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Benzene (Chapter 15) used to be a common laboratory solvent until OSHA (U.S. Occu-pational Safety and Health Administration) placed it on its list of carcinogens. Chemists now use methylbenzene (toluene) instead, which has very similar solvating power but is not carcino- genic. Why not? The reason is the relatively high reactivity of the benzylic hydrogens that renders methylbenzene subject to fast metabolic degrada- tion and extrusion from the body, unlike benzene, which can survive many days embedded in fatty and other tissues. Thus, the benzene ring, though itself quite unreactive because of its aromaticity, appears to activate neighboring bonds or, more generally, affects the chemistry of its substituents. You should not be too surprised by this fi nding, because it is complementary to the conclusions of Chapter 16. There we saw that substituents affect the behavior of benzene. Here we shall see the reverse. How does the benzene ring modify the behavior of neighboring reactive centers? This chapter takes a closer look at the effects exerted by the ring on the reactivity of alkyl substituents, as well as of attached hydroxy and amino functions. We shall see that the behavior of these groups (introduced in Chapters 3, 8, and 21) is altered by the occurrence of resonance. After considering the special reactivity of aryl-substituted (benzylic) carbon atoms, we turn our attention to the preparation and reactions of phenols and benzenamines (anilines). These compounds are found widely in nature and are used in synthetic procedures as precursors to substances such as aspirin, dyes, and vitamins. C H A P T E R T W E N T Y T W O Chemistry of Benzene Substituents Alkylbenzenes, Phenols, and Benzenamines Ciprofl oxacin (“Cipro”) is a synthetic tetrasubstituted benzene derivative used widely to treat bacterial infections, especially of the urinary tract. The photo shows a laboratory worker producing tablets of the drug. OO O N N N F 1020 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 22-1 Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization Methylbenzene is readily metabolized because its methyl C – H bonds are relatively weak with respect to homolytic and heterolytic cleavage. When one of these methyl hydrogens has been removed, the resulting phenylmethyl (benzyl) group, C6H5CH2 (see margin), may be viewed as a benzene ring whose p system overlaps with an extra p orbital located on an attached alkyl carbon. This interaction, generally called benzylic resonance, stabilizes adjacent radical, cationic, and anionic centers in much the same way that overlap of a p bond and a third p orbital stabilizes 2-propenyl (allyl) intermediates (Section 14-1). However, unlike allylic systems, which may undergo transformations at either terminus and give product mixtures (in the case of unsymmetrical substrates), benzylic reactivity is regioselec- tive and occurs only at the benzylic carbon. The reason for this selectivity lies in the dis- ruption of aromaticity that goes with attack on the benzene ring. Benzylic radicals are reactive intermediates in the halogenation of alkylbenzenes We have seen that benzene will not react with chlorine or bromine unless a Lewis acid is added. The acid catalyzes halogenation of the ring (Section 15-9). Br–Br, FeBr3 –HBr Br Br–Br No reaction In contrast, heat or light allows attack by chlorine or bromine on methylbenzene (toluene) even in the absence of a catalyst. Analysis of the products shows that reaction takes place at the methyl group, not at the aromatic ring, and that excess halogen leads to multiple substitution. � CH2Br Br2� CH2H HBr� (Bromomethyl)- benzene CH3 (Chloromethyl)- benzene CH2Cl CHCl2 (Dichloromethyl)- benzene CCl3 (Trichloromethyl)- benzene Cl2, hv �HCl Cl2, hv �HCl Cl2, hv �HCl Each substitution yields one molecule of hydrogen halide as a by-product. As in the halogenation of alkanes (Sections 3-4 through 3-6) and the allylic halogena- tion of alkenes (Section 14-2), the mechanism of benzylic halogenation proceeds through radical intermediates. Heat or light induces dissociation of the halogen molecule into atoms. One of them abstracts a benzylic hydrogen, a reaction giving HX and a phenyl- methyl (benzyl) radical. This intermediate reacts with another molecule of halogen to give the product, a (halomethyl)benzene, and another halogen atom, which propagates the chain process. Phenylmethyl (benzyl) system 2-Propenyl (allyl) system Sole site of reactivity Sites of reactivity REACTION C h a p t e r 2 2 1021 Mechanism of Benzylic Halogenation Initiation: Propagation: X Phenylmethyl (benzyl) radical CH2 �HX j X� j j CH2X XOX CH2OH jOXðð š� X2ðš�Xš� � or h� What explains the ease of benzylic halogenation? The answer lies in the stabilization of the phenylmethyl (benzyl) radical by the phenomenon called benzylic resonance (Figure 22-1). As a consequence, the benzylic C – H bond is relatively weak (DH8 5 87 kcal mol21, 364 kJ mol21); its cleavage is relatively favorable and proceeds with a low activation energy. Inspection of the resonance structures in Figure 22-1 reveals why the halogen attacks only the benzylic position and not an aromatic carbon: Reaction at any but the benzylic carbon would destroy the aromatic character of the benzene ring. Remember: Single-headed (“fi shhook”) arrows denote the movement of single electrons. MECHANISM H H C CH2 or A B C CH2 δ δ δ δ CH2 CH2CH2 Figure 22-1 The benzene p system of the phenylmethyl (benzyl) radical enters into resonance with the adjacent radical center. The extent of delocalization may be depicted by (A) resonance structures, (B) dotted lines, or (C) orbitals. Benzylic cations delocalize the positive charge Reminiscent of the effects encountered in the corresponding allylic systems (Section 14-3), benzylic resonance can affect strongly the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, the 4-methylbenzenesulfonate (tosylate) of 4-methoxyphenylmethanol (4-methoxybenzyl alcohol) reacts with solvent ethanol rapidly via an SN1 mechanism. This reaction is an example of solvolysis, specifi cally ethanolysis, which we described in Chapter 7. Exercise 22-1 For each of the following compounds, draw the structure and indicate where radical halogenation is most likely to occur upon heating in the presence of Br2. Then rank the compounds in approximate descending order of reactivity under bromination conditions. (a) Ethylbenzene; (b) 1,2-diphenylethane; (c) 1,3-diphenylpropane; (d) diphenylmethane; (e) (1-methylethyl)benzene. 2 2 - 1 B e n z y l i c R e s o n a n c e S t a b i l i z a t i o n 1022 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s (4-Methoxyphenyl)methyl 4-methylbenzenesulfonate 1-(Ethoxymethyl)-4-methoxybenzene (A primary benzylic tosylate) CH3O CH2OS CH3O CH2OCH2CH3 O O B B CH3 CH3 CH3CH2OH� HO3S� SN1 Ethanolysis The reason is the delocalization of the positive charge of the benzylic cation through the benzene ring, allowing for relatively facile dissociation of the starting sulfonate. CH3O CH2 L �L� CH3CH2OH CH2 � �� š� CH3O� O ð CH2 CH3O�ð � CH2 Benzylic cation Octet form CH3O�ð CH3O�ð CH2 product CH2 CH3O�� Mechanism of Benzylic Unimolecular Nucleophilic Substitution Several benzylic cations are stable enough to be isolable. For example, the X-ray struc- ture of the 2-phenyl-2-propyl cation (as its SbF6 2 salt) was obtained in 1997 and shows the phenyl – C bond (1.41 Å) to be intermediate in length between those of pure single (1.54 Å) and double bonds (1.33 Å), in addition to the expected planar framework and trigonal arrangement of all sp2 carbons (Figure 22-2), as expected for a delocalized benzylic system. REACTION MECHANISM ANIM ATION ANIMATED MECHANISM: Benzylic nucleophilic substitution H3C H3C 1.41 Å 1.48 Å 1.43 Å 1.37 Å 1.40 Å 114° 116° 122°123° + Figure 22-2 Structure of the 2-phenyl-2-propyl cation. Exercise 22-2 Which one of the two chlorides will solvolyze more rapidly: (1-chloroethyl)benzene, C6H5CHCl, or chloro(diphenyl)methane, (C6H5)2CHCl? Explain your answer. A CH3 The above SN1 reaction is facilitated by the presence of the para methoxy substituent, which allows for extra stabilization of the positive charge. In the absence of this substituent, SN2 processes may dominate. Thus, the parent phenylmethyl (benzyl) halides and sulfonates undergo preferential and unusually rapid SN2 displacements, even under solvolytic condi- tions, and particularly in the presence of good nucleophiles. As in allylic SN2 reactions (Section 14-3), two factors contribute to this acceleration. One is that the benzylic carbon is made relatively more electrophilic by the neighboring sp2-hybridized phenyl carbon C h a p t e r 2 2 1023 (as opposed to sp3-hybridized ones; Section 13-2). The second is stabilization of the SN2 transition state by overlap with the benzene p system (Figure 22-3). CH2Br �CN� Br�� 81% Phenylethanenitrile (Phenylacetonitrile) (Bromomethyl)benzene (Benzyl bromide) CH2CN SN2 ( 100 times faster than SN2 reactions of primary bromoalkanes) ‡ C Nu −� X −� H H Figure 22-3 The benzene p system overlaps with the orbitals of the SN2 transition state at a benzylic center. As a result, the transition state is stabilized, thereby lowering the activation barrier toward SN2 reactions of (halomethyl)benzenes. ANIM ATION ANIMATED MECHANISM: Benzylic nucleophilic substitution Exercise 22-3 Phenylmethanol (benzyl alcohol) is converted into (chloromethyl)benzene in the presence of hydrogen chloride much more rapidly than ethanol is converted into chloroethane. Explain. Resonance in benzylic anions makes benzylic hydrogens relatively acidic A negative charge adjacent to a benzene ring, as in the phenylmethyl (benzyl) anion, is stabilized by conjugation in much the same way that the corresponding radical and cation are stabilized. The electrostatic potential maps of the three species (rendered in the margin at an attenuated scale for optimum contrast) show the delocalized positive (blue) and neg- ative (red) charges in the cation and anion, respectively, in addition to the delocalized electron (yellow) in the neutral radical. CH2� � � CH2 Resonance in Benzylic Anions � H� CH2 CH2ðCH3 k � k � 41pKa The acidity of methylbenzene (toluene; pKa < 41) is therefore considerably greater than that of ethane (pKa < 50) and comparable to that of propene (pKa < 40), which is de- protonated to produce the resonance-stabilized 2-propenyl (allyl) anion (Section 14-4). Consequently, methylbenzene (toluene) can be deprotonated by butyllithium to generate phenylmethyllithium. Phenylmethyl (benzyl) cation Phenylmethyl (benzyl) anion Phenylmethyl (benzyl) radical 2 2 - 1 B e n z y l i c R e s o n a n c e S t a b i l i z a t i o n 1024 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Deprotonation of Methylbenzene � CH3CH2CH2CH2Li CH3 Methylbenzene (Toluene) � CH3CH2CH2CH2H CH2Li Phenylmethyllithium (Benzyllithium) (CH3)2NCH2CH2N(CH3)2, THF, � Exercise 22-4 Which molecule in each of the following pairs is more reactive with the indicated reagents, and why? (a) (C6H5)2CH2 or C6H5CH3, with CH3CH2CH2CH2Li (b) OCH3 or , with NaOCH3 in CH3OH CH2Br OCH3 CH2Cl (c) or , with HCl CH3CHOH NO2 CH3CHOH In Summary Benzylic radicals, cations, and anions are stabilized by resonance with the benzene ring. This effect allows for relatively easy radical halogenations, SN1 and SN2 reactions, and benzylic anion formation. 22-2 Benzylic Oxidations and Reductions Because it is aromatic, the benzene ring is quite unreactive. While it does undergo electrophilic aromatic substitutions (Chapters 15 and 16), reactions that dismantle the aromatic six-electron circuit, such as oxidations and reductions, are much more diffi cult to achieve. In contrast, such transformations occur with comparative ease when taking place at benzylic positions. This sec- tion describes how certain reagents oxidize and reduce alkyl substituents on the benzene ring. Oxidation of alkyl-substituted benzenes leads to aromatic ketones and acids Reagents such as hot KMnO4 and Na2Cr2O7 may oxidize alkylbenzenes all the way to benzoic acids. Benzylic carbon – carbon bonds are cleaved in this process, which usually requires at least one benzylic C – H bond to be present in the starting material (i.e., tertiary alkylbenzenes are inert). CH2CH2CH2CH3 1-Butyl-4-methylbenzene H3C COOH 1,4-Benzenedicarboxylic acid (Terephthalic acid) 80% Complete Benzylic Oxidations of Alkyl Chains HOOC 1. KMnO4, HO�, � 2. H�, H2O �3 CO2 C h a p t e r 2 2 1025 The reaction proceeds through fi rst the benzylic alcohol and then the ketone, at which stage it can be stopped under milder conditions (see margin and Section 16-5). The special reactivity of the benzylic position is also seen in the mild conditions required for the oxidation of benzylic alcohols to the corresponding carbonyl compounds. For example, manganese dioxide, MnO2, performs this oxidation selectively in the presence of other (nonbenzylic) hydroxy groups. (Recall that MnO2 was used in the conversion of allylic alcohols into a,b-unsaturated aldehydes and ketones; see Section 17-4.) OH OH OHH CH3O CH3O CH3O Selective Oxidation of a Benzylic Alcohol with Manganese Dioxide CH3O MnO2, acetone, 25°C, 5 h 94% O Benzylic ethers are cleaved by hydrogenolysis Exposure of benzylic alcohols or ethers to hydrogen in the presence of metal catalysts results in rupture of the reactive benzylic carbon – oxygen bond. This transformation is an example of hydrogenolysis, cleavage of a s bond by catalytically activated hydrogen. 1,2,3,4-Tetra- hydronaphthalene (Tetralin) Not isolated OH 71% 1-Oxo-1,2,3,4-tetrahydro- naphthalene (1-Tetralone) O CrO3, CH3COOH H2O, 21°C , Cleavage of Benzylic Ethers by Hydrogenolysis CH2 ORO H2, Pd–C, 25°C CH2H � HOR Hydrogenolysis is not possible for ordinary alcohols and ethers. Therefore, the phenyl- methyl (benzyl) substituent is a valuable protecting group for hydroxy functions. The following scheme shows its use in part of a synthesis of a compound in the eudesmane class of essential oils, which includes substances of importance in both medicine and perfumery. Exercise 22-5 Write synthetic schemes that would connect the following starting materials with their products. (a) CH3CH2CH3 (b) CH3H3C CH3H3C OO OO OO 2 2 - 2 B e n z y l i c O x i d a t i o n s a n d R e d u c t i o n s 1026 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Because the hydrogenolysis of the phenylmethyl (benzyl) ether in the fi nal step occurs under neutral conditions, the tertiary alcohol function survives untouched. A tertiary butyl ether would have been a worse choice as a protecting group, because cleavage of its carbon – oxygen bond would have required acid (Section 9-8), which may cause dehydration (Section 9-2). In Summary Benzylic oxidations of alkyl groups take place in the presence of permanga- nate or chromate; benzylic alcohols are converted into the corresponding ketones by man- ganese dioxide. The benzylic ether function can be cleaved by hydrogenolysis in a transformation that allows the phenylmethyl (benzyl) substituent to be used as a protecting group for the hydroxy function in alcohols. 22-3 Names and Properties of Phenols Arenes substituted by hydroxy groups are called phenols (Section 15-1). The p system of the benzene ring overlaps with an occupied p orbital on the oxygen atom, a situation result- ing in delocalization similar to that found in benzylic anions (Section 22-1). As one result of this extended conjugation, phenols possess an unusual, enolic structure. Recall that enols are usually unstable: They tautomerize easily to the corresponding ketones because of the relatively strong carbonyl bond (Section 18-2). Phenols, however, prefer the enol to the keto form because the aromatic character of the benzene ring is preserved. O O 1. CH3Li, (CH3CH2)2O 2. H�, H2O (Section 8-8) H CH3% ≥ RO % CCH3 98% % A A CH3 OH H2, Pd–C, CH3CH2OH Deprotection of OR H CH3% ≥ HO % CCH3 98% % A A CH3 OH H CH3% ≥ RO % (Mixture of E and Z isomers) O 93% CH3CHP DMSO Wittig reaction (Section 17-12) P(C6H5)3, H CH3% ≥ RO % CHCH3 94% 1. BH3, THF 2. Oxidation (to alcohol) 3. Oxidation (to ketone) Hydroboration–oxidation* (Section 12-8) Oxidation (Section 8-6) H CH3% ≥ RO % COCH3 99% % O O O H CH3% ≥ 1. NaH, THF 2. C6H5CH2Br O O HO % H CH3% ≥ O O RO % Protection of OH (Section 9-6) CH3COOH, H2O Deprotection of carbonyl group (Section 17-8) 80% (R � C6H5CH2) Phenylmethyl Protection in a Complex Synthesis *While the literature reports the use of special oxidizing agents, in principle 2. H2O2, 2OH and 3. CrO3 would have been satisfactory. C h a p t e r 2 2 1027 Keto and Enol Forms of Acetone and Phenol 2,4-Cyclohexadienone O H �š H OHð šOHðð ð šOH �ð �šOH �š ð� �šOH Phenol K ~ 1013 2-PropenolAcetone K ~ 10�9 Oðð H C H2 H3C CH2H3C šOHð Phenols and their ethers are ubiquitous in nature; some derivatives have medicinal and herbicidal applications, whereas others are important industrial materials. This section fi rst explains the names of these compounds. It then describes an important difference between phenols and alkanols — phenols are stronger acids because of the neighboring aromatic ring. Phenols are hydroxyarenes Phenol itself was formerly known as carbolic acid. It forms colorless needles (m.p. 418C), has a characteristic odor, and is somewhat soluble in water. Aqueous solutions of it (or its methyl-substituted derivatives) are applied as disinfectants, but its main use is for the prep- aration of polymers (phenolic resins; Section 22-6). Pure phenol causes severe skin burns and is toxic; deaths have been reported from the ingestion of as little as 1 g. Fatal poison- ing may also result from absorption through the skin. Substituted phenols are named as phenols, benzenediols, or benzenetriols, according to the system described in Section 15-1, although some common names are accepted by IUPAC (Section 22-8). These substances fi nd uses in the photography, dyeing, and tanning indus- tries. The compound bisphenol A (shown in the margin; see also Chemical Highlight 22-1) is an important monomer in the synthesis of epoxyresins and polycarbonates, materials that are widely employed in the manufacture of durable plastic materials, food packaging, dental sealants, and coatings inside beverage cans (Chemical Highlight 22-1). Phenols containing the higher-ranking carboxylic acid functionality are called hydroxy- benzoic acids. Many have common names. Phenyl ethers are named as alkoxybenzenes. As a substituent, C6H5O is called phenoxy. OH NO2 Cl 4-Chloro- 3-nitrophenol COOH OH 3-Hydroxybenz- oic acid (m-Hydroxybenzoic acid) OH 1,4-Benzenediol (Hydroquinone) OH 1,2,3-Benzenetriol (Pyrogallol) OH OH 4-Methylphenol (p-Cresol) CH3 OH OH Many examples of phenol derivatives, particularly those exhibiting physiological activity, are depicted in this book (e.g., see Chemical Highlights 5-4, 9-1, 21-1, 22-1, and 22-2, and OH OH Bisphenol A CH3C CH3O O 2 2 - 3 N a m e s a n d P r o p e r t i e s o f P h e n o l s 1028 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Phenols are unusually acidic Phenols have pKa values that range from 8 to 10. Even though they are less acidic than carboxylic acids (pKa 5 3 – 5), they are stronger than alkanols (pKa 5 16 – 18). The reason is resonance: The negative charge in the conjugate base, called the phenoxide ion, is stabilized by delocalization into the ring. OH H� � � šð Ošð ð Oð ð � � O Phenoxide ion Acidity of Phenol ð ðOð ð k � k � pKa 10 HO O H3C N O H Capsaicin (Active ingredient in hot pepper, as in jalapeño or cayenne pepper) A E OH OHHO Resveratrol (Cancer chemopreventive from grapes; see also Chemical Highlight 22-1) O O O OH OH OH OH OH OH HO HO ~ HO O 4-(4-Hydroxyphenyl)-2-butanone (Flavor of raspberries) Raspberries. Green tea. Chili peppers. Sections 4-7, 9-11, 15-1, 22-9, 24-12, 25-8, and 26-1). You are very likely to have ingested without knowing it the four phenol derivatives shown here. Epigallocatechin-3 gallate (Cancer chemopreventitive from green tea) C h a p t e r 2 2 1029 The acidity of phenols is greatly affected by substituents that are capable of resonance. 4-Nitrophenol (p-nitrophenol), for example, has a pKa of 7.15. OHšð H� etc. � � Ošð ð Oð ð � k � � N� �O Oð� HK �½ k N� �O Oð� HK �½ k N� �O Oð� HK �½ k �O H �½ k pKa � 7.15 Oð ð N� Oð� �O H �½ kO�½ Oð ð N� �ý E Negative charge delocalized into the nitro group The 2-isomer has similar acidity (pKa 5 7.22), whereas nitrosubstitution at C3 results in a pKa of 8.39. Multiple nitration increases the acidity to that of carboxylic or even mineral acids. Electron-donating substituents have the opposite effect, raising the pKa. O2NNO2 NO2 2,4-Dinitrophenol OH pKa � 4.09 NO2 NO2 2,4,6-Trinitrophenol (Picric acid) OH pKa � 0.25 CH3 4-Methylphenol ( p-Cresol) OH pKa � 10.26 As Section 22-5 will show, the oxygen in phenol and its ethers is also weakly basic, in the case of ethers giving rise to acid-catalyzed cleavage. Exercise 22-6 Why is 3-nitrophenol (m-nitrophenol) less acidic than its 2- and 4-isomers but more acidic than phenol itself? In Summary Phenols exist in the enol form because of aromatic stabilization. They are named according to the rules for naming aromatic compounds explained in Section 15-1. Those derivatives bearing carboxy groups on the ring are called hydroxy- benzoic acids. Phenols are acidic because the corresponding anions are resonance stabilized. Exercise 22-7 Rank in order of increasing acidity: phenol, A; 3,4-dimethylphenol, B; 3-hydroxybenzoic (m-hydroxybenzoic) acid, C; 4-(fl uoromethyl)phenol [p-(fl uoromethyl)phenol], D. 2 2 - 3 N a m e s a n d P r o p e r t i e s o f P h e n o l s 1030 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s CHEMICAL HIGHLIGHT 22-1 Two Phenols in the News: Bisphenol A and Resveratrol This section mentions two phenols as examples of chemicals to which you have potentially frequent exposure: bisphenol A (p. 1027) and resveratrol (p. 1028). Bisphenol A is the essential ingredient of the familiar polycarbonate plastics used in clear baby bottles, instant-formula cans, dental sealants, the linings of food and beverage containers, and reusable plastic bottles. More than 2 billion pounds are made each year in the United States alone, and three times as much are made worldwide. Yet there is continuing controversy surrounding this monomer, fi rst made in 1891— a controversy that illustrates the diffi culty of interpreting scientifi c data in terms of human risk assessment. The problem is that bisphenol A behaves as an estrogen mimic in animals. The monomer has been found to leach from the plastic, at increasing rates when heated, such as in a microwave oven. For example, a study carried out in 2003 showed that even very low levels of bisphenol A (about 20 ppb) caused chromosome aberrations in developing mouse eggs. These amounts are in the same range as are encountered in human blood and urine. Because the pro- cesses through which human and mouse eggs are prepared for fertilization are very similar, the study is a cause for concern, although it does not prove that humans are at risk. Moreover, in other studies carried out with adult rats, bisphenol A appeared to have no adverse effects on repro- duction and development. A critique of these fi ndings, dated 2008, pointed out that fetuses and newborns lack the liver enzyme needed to detoxify the chemical, again raising the specter of adverse effects on children. As a report by the National Institute of Environmental Health Sciences stresses, the discrepancies in studies of this type may be due to the use of different strains of animals, varying degrees of exposure and differing background levels of estrogenic pollutants, dosing regimens, and housing of the animals A A B O n O CC CH3 CH3 O O Carbonate unit O O Bisphenol A unit Polycarbonate plastics A polycarbonate plastic bottle in action. 22-4 Preparation of Phenols: Nucleophilic Aromatic Substitution Phenols are synthesized quite differently from the way in which ordinary substituted ben- zenes are made. Direct electrophilic addition of OH to arenes is diffi cult because of the scarcity of reagents that generate an electrophilic hydroxy group, such as HO1. Instead, phenols are prepared by formal nucleophilic displacement of a leaving group from the arene ring by hydroxide, HO2, reminiscent of, but mechanistically quite different from, the synthesis of alkanols from haloalkanes. This section considers the ways in which this transformation may be achieved. Nucleophilic aromatic substitution may follow an addition – elimination pathway Treatment of 1-chloro-2,4-dinitrobenzene with hydroxide replaces the halogen with the nucleophile, furnishing the corresponding substituted phenol. Other nucleophiles, such C h a p t e r 2 2 1031 (singly versus group). Remember that we are talking about extremely small concentrations of a biologically active com- pound (parts per billion!) that affect only a certain percent- age of animals and to differing degrees. And then there are the big questions of the extent to which animal studies are relevant to humans and whether there is a threshold level of exposure that humans can tolerate because of the presence of evolutionary natural detoxifi cation mechanisms. In 2007 and 2008, two government-convened groups reiterated that human exposure is at a level that causes adverse effects in humans and that there is concern about prenatal and early- childhood exposure. As a result, both Canada and the U.S. Congress began moving toward a ban of bisphenol A in children’s products, and some retailers, such as Wal-Mart and Toys“R”Us, are phasing out their sale. Concurrently, industrial producers are replacing polycarbonate plastics with other polymers. The issues surrounding the potentially harmful effects of chemicals in the environment are equally relevant with respect to their potentially benefi cial effects. An example is resveratrol. This compound has been used in traditional medicine to treat conditions of the heart and liver, and recently scientists took an interest in its physiological prop- erties. It is present in various plants and foods, such as eucalyptus, lily, mulberries, peanuts, and, most prominently, in the skin of white and, especially, red grapes, where it is found in concentrations of 50–100 mg/g. The compound is a chemical weapon against invading organisms, such as fungi. Grapes are used for wine making, so resveratrol occurs in red wine, at levels of up to 160 mg per ounce. Studies have suggested that the regular consumption of red wine reduces the incidence of coronary heart disease, a fi nding described as the “French paradox,” namely, the low incidence of heart problems in France despite the relatively high-fat diet. Resveratrol may be the active species, as recent research has indicated its potentially benefi cial cardiovascular effects, including its action as an antioxidant, which inhibits lipid peroxidation (Section 22-9), and as an antiplatelet agent (Chemical Highlight 22-2), which prevents atherosclerosis. Other investigations have shown that the molecule is also an active antitumor agent, involved in retarding the initia- tion, promotion, and progression of certain cancers, with seemingly minimal toxicity. Perhaps most interesting, it was discovered that resveratrol signifi cantly extended the lifespan of certain species of yeast, worm, the fruit fl y, and fi sh. Along similar lines, it canceled the life-shortening effects of a high-fat diet in mice. As a result of these promising discoveries, resveratrol has been hailed by manufacturers as the “French paradox in a bottle” and pushed for wide-scale consumption. However, experts advise caution. For example, little is known about its metabolism and how it affects the liver, the above results are derived in large part from in vitro experiments, and, like bisphenol A, it has estrogen-like physiological effects, enhancing the growth of breast cancer cells. For the time being, an occasional glass of red wine may be the best course of action, if any! Resveratrol protects grapes from fungus, such as that shown here. as alkoxides or ammonia, may be similarly employed, forming alkoxyarenes and arenamines, respectively. Processes such as these, in which a group other than hydro- gen is displaced from an aromatic ring, are called ipso substitutions (ipso, Latin, on itself). The products of these reactions are intermediates in the manufacture of useful dyes. NO2 � Nu� NO2 Cl ð NO2 � Cl� NO2 Nu Nucleophilic Aromatic Ipso Substitution Electron withdrawing Ipso position REACTION 2 2 - 4 N u c l e o p h i l i c A r o m a t i c S u b s t i t u t i o n 1032 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s The transformation is called nucleophilic aromatic substitution. The key to its suc- cess is the presence of one or more strongly electron-withdrawing groups on the benzene ring located ortho or para to the leaving group. Such substituents stabilize an intermedi- ate anion by resonance. In contrast with the SN2 reaction of haloalkanes, substitution in these reactions takes place by a two-step mechanism, an addition – elimination sequence similar to the mechanism of substitution of carboxylic acid derivatives (Sections 19-7 and 20-2). Mechanism of Nucleophilic Aromatic Substitution Step 1. Addition (facilitated by resonance stabilization) �N Cl � � NO2 EO � Nu �ð � Oð�š � Oð�š ðO� � ðð ð Oðð š� O �ðš� O �ðš� EO �ðš� EO �ðš� B k � k NO2 NO2 �N O NO2 Nu Cl NO2 ðð �š NO2 �N ONu Cl Nu Cl Nu ClNu Cl The negative charge is strongly stabilized by resonance involving the ortho- and para-NO2 groups. N�NE HN � E HN � Oð� NO2 NO2 Cl NO2 � NH4Cl NO2 85% 2,4-Dinitrobenzenamine (2,4-Dinitroaniline) NH2 NH3, �ð NO2 Na2CO3, HOH, 100°C NO2 90% 1-Chloro-2,4- dinitrobenzene 2,4-Dinitrophenol Cl NO2 � NaCl NO2 OH MECHANISM ANIM ATION ANIMATED MECHANISM Nucleophilic aromatic substitution C h a p t e r 2 2 1033 Step 2. Elimination (only one resonance structure is shown) N� NO2 EO �ð Oðð š� B N� EO � Cl � �ð Oðð š� ððš� Clðš� BNu �k NO2 Nu In the fi rst and rate-determining step, ipso attack by the nucleophile produces an anion with a highly delocalized charge, for which several resonance structures may be written, as shown. Note the ability of the negative charge to be delocalized into the electron-withdrawing groups. In contrast, such delocalization is not possible in 1-chloro-3,5-dinitrobenzene, in which these groups are located meta; so this compound does not undergo ipso substitution under the conditions employed. NO2 Cl O2N NO2O2N � k NO2O2NNO2O2N Meta-NO2 groups do not provide resonance stabilization of the negative charge. � � �k Nu �ð Nu Cl Nu Cl Nu Cl In the second step, the leaving group is expelled to regenerate the aromatic ring. The reactivity of haloarenes in nucleophilic substitutions increases with the nucleophilicity of the reagent and the number of electron-withdrawing groups on the ring, particularly if they are in the ortho and para positions. Exercise 22-8 Write the expected product of reaction of 1-chloro-2,4-dinitrobenzene with NaOCH3 in boiling CH3OH. 2 2 - 4 N u c l e o p h i l i c A r o m a t i c S u b s t i t u t i o n Exercise 22-9 Working with the Concepts: Using Nucleophilic Aromatic Substitution in Synthesis Ofl oxacin, an antibiotic of the quinolone class (Section 25-7), is used in the treatment of infections of the respiratory and urinary tract, the eyes, the ears, and skin tissue. The quinolone antibiotics are yet another alternative in the ongoing battle against penicillin (and other drug)-resistant bacteria. The fi nal three steps in the synthesis of ofl oxacin are shown below, the last of which is a simple ester hydrolysis (Section 20-4). Formulate mechanisms for the other two transformations from A to B and then to C. CH3 F F F OH F HN H3C O O O F O F O O Na�H�, 70–90°C O N CH3 CH3 F COH N O NaOH, H2O O N N N N H CH3 A , toluene, 100�C E B O CH3 CH3 F O N O O O N NE A OfloxacinC B Strategy Taking an inventory of the topological changes and of the nature of the bonds formed and bonds broken, we note that B and C are the result of two successive intra- and one intermolecular nucleophilic aromatic substitutions, respectively. The fi rst steps are ring closures and therefore espe- cially favored for entropic and enthalpic reasons. (Formation of six-membered rings is a favorable reaction, see Sections 9-6 and 17-7.) The aromatic ring in A should be highly activated with respect to nucleophilic attack, because it bears four inductively electron-withdrawing fl uorine substitu- ents, in addition to the resonance-based electron-withdrawing carbonyl group. To propose a mechanism, let’s look at the individual steps. Solution • In the conversion of A to B, the strong base Na1H2 is employed. Where are the acidic sites in A? Most obvious is the hydroxy function (pKa , 15–16, Section 8-3, Table 8-2), which will certainly be deprotonated. • What about the amino group? Closer inspection of structure A shows that the nitrogen is connected via a double bond to two carbonyl moieties. Thus, the function is resonance stabilized in much the same way as an ordinary amide (but more so): F F F OH F HN H3C O š Oð ð Oð ðOð ð F F F OH F H3C O š�Oð ð Oð ð HN� F F F OH F H3C O š�Oð ð HN� Resonance in A Consequently, it should be relatively acidic, even more acidic than an amide (pKa < 22, Section 20-7). It is therefore likely that A is doubly deprotonated before attacking the benzene nucleus. Nucleophilic aromatic substitution is then readily formulated to assemble the fi rst new ring. In the scheme below, only one—the dominating—resonance form of the anion resulting from nucleophilic attack on the arene is shown, that in which the negative charge is delocalized onto the carbonyl oxygen. � š š � Oð F O F F H3C OðO N ð š š � Oð š � F O F F H3C OðO N ð š š � O šš ð Fðð š š š �� Fðð F F F F N H3C O š Oð ð O ð First Ring Closure The second nucleophilic substitution by alkoxide is unusual, inasmuch as it has to occur at a position meta to the carbonyl function. Therefore, the intermediate anion is not stabilized by resonance, but solely by inductive effects. The regioselectivity is controlled by strain: The alkoxide cannot “reach” the more favorable carbon para to the carbonyl. F O F NF OO �Oðð CH3 š š Fðð š š š �� Fðð F O N O F OO � ð ð CH3 F O NF OO O CH3 B Second Ring Closure to B The conversion of B to C features an intermolecular substitution with an amine nucleophile. Of the two carbons bearing the potential fl uo- ride leaving groups, the one para to the carbonyl is picked, because of the resonance stabilization of the resulting intermediate anion. The initial product is formed as the ammonium salt, from which the free base C is liberated by basic work-up (Chemical Highlight 21-2). š š Fðð š F O NN H N F � OO OCH3 CH3 CH3 �HF (on basic work-up) CH3 B C A A A ðð F H O NN OO ON ð ð ð ððš � � 1034 C h a p t e r 2 2 1035 Haloarenes undergo substitution through benzyne intermediates Haloarenes devoid of electron-withdrawing substituents do not undergo simple ipso substi- tution. Nevertheless, when haloarenes are treated with nucleophiles that are also strong bases, if necessary at highly elevated temperatures, they convert to products in which the halide has been replaced by the nucleophile. For example, if exposed to hot sodium hydroxide followed by neutralizing work-up, chlorobenzene furnishes phenol. Cl Chlorobenzene OH � NaCl Phenol 1. NaOH, H2O, 340°C, 150 atm 2. H�, H2O Treatment with potassium amide results in benzenamine (aniline). Cl NH2 � KCl Benzenamine (Aniline) 1. KNH2, liquid NH3 2. H�, H2O It is tempting to assume that these substitutions follow a mechanism similar to that formulated for nucleophilic aromatic ipso substitution earlier in this section. However, when the last reaction is performed with radioactively labeled chlorobenzene (14C at C1), a very curious result is obtained: Only half of the product is substituted at the labeled carbon; in the other half, the nitrogen is at the neighboring position. Cl � Chlorobenzene-1-14C 14C label KNH2, liquid NH3 �KCl NH2 Benzenamine-1-14C 50% Benzenamine-2-14C 50% NH2 Exercise 22-10 Try It Yourself Propose a mechanism for the following conversion. Considering that the fi rst step is rate deter- mining, draw a potential-energy diagram depicting the progress of the reaction. (Hint: This is a nucleophilic aromatic substitution.) NO2 NO2 O� SO2H3C O SO2�H3C REACTION 2 2 - 4 N u c l e o p h i l i c A r o m a t i c S u b s t i t u t i o n 1036 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Direct substitution does not seem to be the mechanism of these reactions. What, then, is the answer to this puzzle? A clue is the attachment of the incoming nucleophile only at the ipso or at the ortho position relative to the leaving group. This observation can be accounted for by an initial base-induced elimination of HX from the benzene ring, a process reminiscent of the dehydrohalogenation of haloalkenes to give alkynes (Section 13-4). In the present case, elimination is not a concerted process, but rather takes place in a sequen- tial manner, deprotonation preceding the departure of the leaving group (step 1 of the mechanism shown). Both stages in step 1 are diffi cult, with the second being worse than the fi rst. Why is that? With respect to the initial anion formation, recall (Section 11-3) that the acidity of Csp2 – H is very low (pKa < 44), and the same is true in general for phenyl hydrogens. The presence of the adjacent p system of benzene does not help, because the negative charge in the phenyl anion resides in an sp2 orbital that is perpendicular to the p frame and is therefore incapable of resonance with the double bonds in the six-membered ring. Thus, deprotonation of the haloarene requires a strong base. It takes place ortho to the halogen, because the halogen’s inductive electron-withdrawing effect acidifi es this position relative to the others. Although deprotonation is not easy, the second stage of step 1, subsequent elimination of X2, is even more diffi cult because of the highly strained structure of the resulting reactive species, called 1,2-dehydrobenzene or benzyne. Step 1. Elimination occurs stepwise Step 2. Addition occurs to both strained carbons NH2 X H Phenyl anion (Intermediate) Benzyne (Reactive intermediate; not isolated) X + X− − − − − NH2 − HNH2 H — NH2− NH2 − − NH2 − − NH2 Mechanism of Nucleophilic Substitution of Simple Haloarenes NH2 NH2 H H — NH2− NH2 NH2 H pKa ~ 44 sp2 - Hybrid orbital is perpendicular to aromatic systemπ Highly strained triple bond Why is benzyne so strained? Recall that alkynes normally adopt a linear structure, a consequence of the sp hybridization of the carbons making up the triple bond (Sec- tion 13-2). Because of benzyne’s cyclic structure, its triple bond is forced to be bent, rendering it unusually reactive. Thus, benzyne exists only as a reactive intermediate under these conditions, being rapidly attacked by any nucleophile present. For example (step 2), amide ion or even ammonia solvent can add to furnish the product benzenamine (aniline). Because the two ends of the triple bond are equivalent, addition can take place at either carbon, explaining the label distribution in the benzenamine obtained from 14C-labeled chlorobenzene. Benzyne is too reactive to be isolated and stored in a bottle, but it can be observed spectroscopically under special conditions. Irradiation of benzocyclobutenedione at 77 K ANIM ATION ANIMATED MECHANISM: Nucleophilic aromatic substitution via benzynes MECHANISM C h a p t e r 2 2 1037 (21968C) in frozen argon (m.p. 5 21898C) produces a species whose IR and UV spectra are assignable to benzyne, formed by loss of two molecules of CO. Generation of Benzyne, a Reactive Intermediate O Benzocyclobutene-1,2-dione O � 2 COhv, 77 K Although benzyne is usually represented as a cycloalkyne (Figure 22-4A), its triple bond exhibits an IR stretching frequency of 1846 cm21, intermediate between the values for nor- mal double (cyclohexene, 1652 cm21) and triple (3-hexyne, 2207 cm21) bonds. The 13C NMR values for these carbons (d 5 182.7 ppm) are also atypical of pure triple bonds (Sec tion 13-3), indicating a considerable contribution of the cumulated triene (Section 14-5) resonance form (Figure 22-4B). The bond is weakened substantially by poor p orbital overlap in the plane of the ring. C 1.43 Å 1.42 Å Poor overlap BA 1.24 Å 1.40 Å Figure 22-4 (A) The orbital picture of benzyne reveals that the six aromatic p electrons are located in orbitals that are perpendicular to the two additional hybrid orbitals making up the distorted triple bond. These hybrid orbitals overlap only poorly; therefore, benzyne is highly reactive. (B) Resonance in benzyne. (C) The electrostatic potential map of benzyne shows electron density (red) in the plane of the six-membered ring at the position of the distorted sp-hybridized carbons. Exercise 22-11 1-Chloro-4-methylbenzene (p-chlorotoluene) is not a good starting material for the preparation of 4-methylphenol (p-cresol) by direct reaction with hot NaOH, because it forms a mixture of two products. Why does it do so, and what are the two products? Exercise 22-12 Explain the regioselectivity observed in the following reaction. (Hint: Consider the effect of the methoxy group on the selectivity of attack by amide ion on the intermediate benzyne.) OCH3 Br NH2 OCH3 � H2N Major OCH3 Minor KNH2, liquid NH3 �KBr 2 2 - 4 N u c l e o p h i l i c A r o m a t i c S u b s t i t u t i o n 1038 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Phenols are produced from arenediazonium salts The most general traditional laboratory procedure for making phenols is from arenamines through their arenediazonium salts, ArN2 1X2. Recall that primary alkanamines can be N-nitrosated but that the resulting species rearrange to diazonium ions, which are unstable — they lose nitrogen to give carbocations (Section 21-10). In contrast, primary benzenamines (anilines) are attacked by cold nitrous acid, in a reaction called diazotization, to give relatively stable, isolable, although still reactive arenediazonium salts. Compared to their alkanediazonium counterparts, these species enjoy resonance stabilization and are prevented from undergoing immediate N2 loss by the high energy of the resulting aryl cations (to be discussed in more detail in Section 22-10). When arenediazonium ions are gently heated in water, nitrogen is evolved and the resulting aryl cations are trapped extremely rapidly by the solvent to give phenols. Decomposition of Arenediazonium Salts in Water to Give Phenols R N+ N Aryl cation −N2 Δ −H+ HOH R R OH+ In these reactions, the “super” leaving group N2 accomplishes what halides are only able to do when attached to a highly electron-defi cient benzene nucleus (nucleophilic aro- matic substitution) or under extreme conditions (through benzyne intermediates), namely, replacement by hydroxide. The three mechanisms are completely different. In nucleo- philic aromatic substitution, the nucleophile attacks prior to departure of the leaving group. In the benzyne mechanism, the nucleophile acts initially as a base, followed by extrusion of the leaving group and subsequent nucleophilic attack on the strained triple bond. In the arenediazonium-ion decomposition, the leaving group exits fi rst, followed by trapping by water. The utility of this phenol synthesis is apparent when you recall that arenamines are derived from nitroarenes by reduction, and nitroarenes are made from other arenes by electrophilic aromatic substitution (Chapters 15 and 16). Therefore, retrosynthetically (Sec- tion 8-9), we can picture the hydroxy group in any position of a benzene ring that is subject to electrophilic nitration. R Diazotization NH2 R Arenediazonium ion N� N NaNO2, H�, H2O, 0°C c š C h a p t e r 2 2 1039 Retrosynthetic Connection of Phenols to Arenes NH2OH OH D(X) NH2 D(X) NO2 NO2 D(X) D(X) Donor-activated or halogen-bearing arene NH2 NO2OH A A A A Acceptor-deactivated arene Two examples are depicted below. 70% (separated from the ortho isomer by crystallization) 98% 70% NO2 BrBr NH2 Br HNO3, H2SO4 CH3COOH, Fe HNO3, H2SO4, 20�C CH3O 80% N2 �Cl� Br 4-Bromophenol (p-Bromophenol) OH Br NaNO2, HCl, H2O, 0�C NaNO2, H2SO4, H2O, 0�C H2, Ni 80�C 80�C CH3O NO2 85% CH3O NH2 82% CH3O OH CH3O N2 � HSO4� 87% 1-(3-Hydroxyphenyl)ethanone (m-Hydoxyacetophenone) Exercise 22-13 ortho-Benzenediazoniumcarboxylate A (made by diazotization of 2-aminobenzoic acid, Prob- lem 20-59) is explosive. When warmed in solution with trans,trans-2,4-hexadiene, it forms com- pound B. Explain by a mechanism. (Hint: Two other products are formed, both of which are gases.) CH3 CH3 CH3H3CN � � C O O A B B E ð Nð ðð š� q 0 % Exercise 22-14 Propose a synthesis of 4-(phenylmethyl)phenol (p-benzylphenol) from benzene. (Caution: Remember that Friedel-Crafts reactions do not work with deactivated arenes.) 2 2 - 4 N u c l e o p h i l i c A r o m a t i c S u b s t i t u t i o n 1040 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Phenols can be made from haloarenes by Pd catalysis While we have seen that ordinary halobenzenes are resilient to reaction with hydroxide, they undergo such nucleophilic displacements in the presence of Pd salts and added phos- phine ligands PR3. X OH KOH, Pd catalyst, PR3, 100�C Pd-Catalyzed Phenol Synthesis from Haloarenes The reaction is general for substituted benzenes, providing a complement to the diazonium method described above. OCH3 Cl OCH3 OH 90% 4-Methoxyphenol (p-Methoxyphenol) 98% 1-(3-Hydroxyphenyl)ethanone (m-Hydoxyacetophenone) KOH, Pd catalyst, PR3, 100�C KOH, Pd catalyst, PR3, 100�C CH3 Br O CH3 OH O The mechanism is related to that of the Heck and other Pd-catalyzed reaction (Section 13-9; Chemical Highlight 13-1). As shown in a simplifi ed manner below, it begins by insertion of the metal into the aryl halide bond, exchange of the halide for hydroxide, and extrusion of the fi nal product with regeneration of the catalyst. Mechanism of the Pd-Catalyzed Phenol Synthesis from Haloarenes X OH X Pd OH Pd Pd HO� �X� �Pd Similar substitutions can be carried out with alkoxides to give phenol ethers, and with amines, including ammonia, to furnish benzenamines. OCH3 NH2Br 98% 3-Methoxy-N-(2-methylpropyl)benzenamine 3-Methoxy-N-(2-methylpropyl)aniline OCH3 N H � Pd catalyst, PR3, 100�C Br NH2 89% 2-(1-Methylethyl)benzenamine (o-Isopropylaniline) NH3 (14 atm), Pd catalyst, PR3, 90�C C h a p t e r 2 2 1041 In Summary When a benzene ring bears enough strongly electron-withdrawing substitu- ents, nucleophilic addition to give an intermediate anion with delocalized charge becomes feasible, followed by elimination of the leaving group (nucleophilic aromatic ipso substitu- tion). Phenols result when the nucleophile is hydroxide ion, arenamines (anilines) when it is ammonia, and alkoxyarenes when alkoxides are employed. Very strong bases are capable of eliminating HX from haloarenes to form the reactive intermediate benzynes, which are subject to nucleophilic attack to give substitution products. Phenols may also be prepared by decomposition of arenediazonium salts in water and by Pd-catalyzed hydroxylations of haloarenes. 22-5 Alcohol Chemistry of Phenols The phenol hydroxy group undergoes several of the reactions of alcohols (Chapter 9), such as protonation, Williamson ether synthesis, and esterifi cation. The oxygen in phenols is only weakly basic Phenols are not only acidic but also weakly basic. They (and their ethers) can be protonated by strong acids to give the corresponding phenyloxonium ions. Thus, as with the alkanols, the hydroxy group imparts amphoteric character (Section 8-3). However, the basicity of phenol is even less than that of the alkanols, because the lone electron pairs on the oxygen are delocalized into the benzene ring (Sections 16-1 and 16-3). The pKa values for pheny- loxonium ions are, therefore, lower than those of alkyloxonium ions. šð � OH šCH3OH � H� � H� šO š H H CH3O � i f H pKa � �6.7pKa � �2.2 H �H E pKa Values of Methyl- and Phenyloxonium Ion Exercise 22-15 How would you make the following phenols from the given starting materials? (Hint: Consult Chapters 15 and 16.) (a) OH from CF3 CF3 (b) from OCH3 NO2 NO2HO OCH3 (c) from OH 2 2 - 5 A l c o h o l C h e m i s t r y o f P h e n o l s OOH A “Green” Industrial Phenol Synthesis H3PO4Friedel-Crafts alkylation H2SO4 O2 (air), ROOR � Cheap starting materials OH � Value- added products O Although the preceding methods are valuable in the preparation of specifi cally substituted phenols, the parent compound is made industrially by the air oxidation of (1-methylethyl)- benzene (isopropylbenzene or cumene; see also Exercise 15-27) to the benzylic hydroper- oxide and its subsequent decomposition with acid (margin). The “by-product” acetone is valuable in its own right and makes this process highly cost effective, quite apart from the environmentally benign use of air as an oxidant. 1042 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Unlike secondary and tertiary alkyloxonium ions derived from alcohols, phenyloxonium derivatives do not dissociate to form phenyl cations, because such ions have too high an energy content (see Section 22-10). The phenyl – oxygen bond in phenols is very diffi cult to break. However, after protonation of alkoxybenzenes, the bond between the alkyl group and oxygen is readily cleaved in the presence of nucleophiles such as Br2 or I2 (e.g., from HBr or HI) to give phenol and the corresponding haloalkane. �HBr, COOH OCH3 CH3Br COOH OH 90% 3-Methoxybenzoic acid (m-Methoxybenzoic acid) 3-Hydroxybenzoic acid (m-Hydroxybenzoic acid) � Alkoxybenzenes are prepared by Williamson ether synthesis The Williamson ether synthesis (Section 9-6) permits easy preparation of many alkoxy- benzenes. The phenoxide ions obtained by deprotonation of phenols (Section 22-3) are good nucleophiles. They can displace the leaving groups from haloalkanes and alkyl sulfonates. NaOH, H2O �NaBr, �HOH OH OCH2CH2CH3 CH3CH2CH2Br ClCl 63% 3-Chlorophenol (m-Chlorophenol) 1-Chloro-3-propoxybenzene (m-Chlorophenyl propyl ether) � Esterifi cation leads to phenyl alkanoates The reaction of a carboxylic acid with a phenol (Section 19-9) to form a phenyl ester is endothermic. Therefore, esterifi cation requires an activated carboxylic acid derivative, such as an acyl halide or a carboxylic anhydride. NaOH, H2O �NaCl, �HOH OH OCCH2CH3 CH3CH2CCl O 4-Methylphenol (p-Cresol) Propanoyl chloride � B O B CH3 4-Methylphenyl propanoate (p-Methylphenyl propanoate) CH3 Exercise 22-16 Why does cleavage of an alkoxybenzene by acid not produce a halobenzene and the alkanol? Exercise 22-17 Explain why, in the preparation of acetaminophen (Chemical Highlight 22-2), the amide is formed rather than the ester. (Hint: Review Section 6-8.) C h a p t e r 2 2 1043 In Summary The oxygen in phenols and alkoxybenzenes can be protonated even though it is less basic than the oxygen in the alkanols and alkoxyalkanes. Protonated phenols and their derivatives do not ionize to phenyl cations, but the ethers can be cleaved to phenols and haloalkanes by HX. Alkoxybenzenes are made by Williamson ether synthesis, aryl alkanoates by acylation. 2 2 - 5 A l c o h o l C h e m i s t r y o f P h e n o l s CHEMICAL HIGHLIGHT 22-2 Aspirin: A Phenyl Alkanoate Drug The year 1997 marked the 100th birthday of the synthesis of the acetic ester of 2-hydroxybenzoic acid (salicylic acid); namely, 2-acetyloxybenzoic acid (acetylsalicylic acid), better known as aspirin (see Section 19-13 and Chapter 16 Opening). Aspirin is the fi rst drug that was clinically tested before it was marketed, in 1899. More than 100 billion tablets of aspirin per year are taken by people throughout the world to relieve headaches and rheumatoid and other pain, to control fever, and to treat gout and arthritis. Production capacity in the United States alone is 10,000 tons per year. Salicylic acid (also called spiric acid, hence the name aspirin [“a” for acetyl]) in extracts from the bark of the willow tree or from the meadowsweet plant had been used since ancient times (see Chapter 16 Opening) to treat pain, fever, and swelling. This acid was fi rst isolated in pure form in 1829, then synthesized in the laboratory, and fi nally produced on a large scale in the nineteenth century and prescribed as an analgesic, antipyretic, and anti-infl ammatory drug. Its bitter taste and side effects, such as mouth irritation and gastric bleeding, prompted the search for better deriva- tives that resulted in the discovery of aspirin. In the body, aspirin functions as a precursor of salicylic acid, which irreversibly inhibits the enzyme cyclooxygenase. This enzyme causes the production of prostaglandins (see Chemical Highlight 11-1 and Section 19-13), molecules that in turn are infl ammatory and pain producing. In addition, one of them, thromboxane A2, aggregates blood platelets, necessary for the clotting of blood when injury occurs. This same pro- cess is, however, undesirable inside arteries, causing heart attacks or brain strokes, depending on the location of the clot. Indeed, a large study conducted in the 1980s showed that aspirin lowered the risk of heart attacks in men by almost 50% and reduced the mortality rate during an actual attack by 23%. Many other potential applications of aspirin are under investigation, such as in the treatment of pregnancy-related complications, viral infl ammation in AIDS patients, dementia, Alzheimer’s disease, and cancer. Despite its popularity, aspi- rin can have some serious side effects: It is toxic to the liver, prolongs bleeding, and causes gastric irritation. It is sus- pected as the cause of Reye’s syndrome, a condition that leads to usually fatal brain damage. Because of some of these drawbacks, many other drugs compete with aspirin, particu- larly in the analgesics market, such as naproxen, ibuprofen, and acetaminophen (see the beginning of Chapter 16). Acetaminophen, better known as Tylenol, is prepared from 4-aminophenol by acetylation. OH 2-Hydroxybenzoic acid (o-Hydroxybenzoic acid, salicylic acid) COOH 2-Acetyloxybenzoic acid (o-Acetoxybenzoic acid, acetylsalicylic acid, aspirin) CH3COCCH3, H�, � �CH3COOH O B O B OCCH3 OCOOH B Aspirin prevents heart attacks by minimizing the formation of blood clots in the coronary arteries. The photograph shows such a blood clot (orange) in the left main pulmonary artery of a patient (see also p. 904). NH2 4-Aminophenol ( p-Aminophenol) OH HNCCH3 O N-(4-Hydroxyphenyl)acetamide [N-( p-Hydroxyphenyl)acetamide, acetaminophen, Tylenol] OH CH3COCCH3, CH3COOH �CH3COOH O B O B B 1044 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 22-6 Electrophilic Substitution of Phenols The aromatic ring in phenols is also a center of reactivity. The interaction between the OH group and the ring strongly activates the ortho and para positions toward electrophilic sub- stitution (Sections 16-1 and 16-3). For example, even dilute nitric acid causes nitration. HNO3, CHCl3, 15�C OH 61%26% 2-Nitrophenol (o-Nitrophenol) OHOH 4-Nitrophenol ( p-Nitrophenol) � NO2 NO2 Friedel-Crafts acylation of phenols is complicated by ester formation and is better car- ried out on ether derivatives of phenol (Section 16-5), as shown in the margin. Phenols are halogenated so readily that a catalyst is not required, and multiple halogena- tions are frequently observed (Section 16-3). As shown in the following reactions, tribromi- nation occurs in water at 208C, but the reaction can be controlled to produce the monohalogenation product through the use of a lower temperature and a less polar solvent. 3 Br–Br, H2O, 20�C �3 HBr Br–Br, CHCl3, 0�C �HBr OH 100% 2,4,6-TribromophenolPhenol OHOH 4-Methylphenol ( p-Cresol) but CH3 Br Br Br 80% 2-Bromo-4-methylphenol OH Br CH3 Halogenation of Phenols Electrophilic attack at the para position is frequently dominant because of steric effects. However, it is normal to obtain mixtures resulting from both ortho and para substitutions, and their compositions are highly dependent on reagents and reaction conditions. Exercise 22-18 Friedel-Crafts methylation of methoxybenzene (anisole) with chloromethane in the presence of AlCl3 gives a 2;1 ratio of ortho;para products. Treatment of methoxybenzene with 2-chloro- 2-methylpropane (tert-butyl chloride) under the same conditions furnishes only 1-methoxy-4- (1,1-dimethylethyl) benzene (p-tert-butylanisole). Explain. (Hint: Review Section 16-5.) Exercise 22-19 Working with the Concepts: Devising Synthetic Strategies Starting with Substituted Phenols Proparacaine is a local anesthetic that is used primarily to numb the eye before minor surgical procedures, such as the removal of foreign objects or stitches. Show how you would make it from 4-hydroxybenzenecarboxylic acid. O O 4-Hydroxybenzenecarboxylic acid Proparacaine H2NOH HO O N O 70% OCH3 Methoxybenzene (Anisole) � H OCH3 1-(4-Methoxyphenyl)ethanone (p-Methoxyacetophenone) C B CH3CCl O HKO CH3 AlCl3, CS2�HCl C h a p t e r 2 2 1045 Strategy Inspection of the structures of both the starting material and the product reveals three structural modifi cations: (1) An amino group is introduced into the benzene core, suggesting a nitration-reduction sequence (Section 16-5) that relies on the ortho-directing effect of the hydroxy substituent: (2) the phenol function is etherifi ed, best done by Williamson synthesis (Section 22-5): (3) the carboxy function is esterifi ed with the appropriate amino alcohol (Section 20-2). What is the best sequence in which to execute these manipulations? To answer this question, consider the possible interference of the various functions with the suggested reaction steps. Solution • Modifi cation 1 could be carried out with the starting material, resulting in the corresponding amino acid. However, “unmasking” the amino group (from its precursor nitro) early bodes trouble with steps 2 and 3, because amines are better nucleophiles than alcohols. There- fore, attempted etherifi cation of the aminated phenol will lead to amine alkylation (Section 21-5). Similarly, attempts to effect ester forma- tion in the presence of an amine function will give rise to amide generation (Section 19-10). However, early introduction of the nitro group and maintaining it as such until the other functions are protected looks good. • For modifi cation 2, Williamson ether synthesis in the presence of a carboxy group should be fi ne, because the carboxylate ion is a poorer nucleophile than phenoxide ion (Section 6-8). • The preceding protection of the phenolic function is important to the success of modifi cation 3, ester formation with the amino alcohol, especially if we want to activate the carboxy group as an acyl chloride. • Putting it all together, a possible (and, indeed, the literature) synthesis of proparacaine is as follows: O proparacaine OH HO O O2N O2N HNO3 O N O O O2N OO OH OH HO H2, Pd-C NaOH, CH3CH2CH2Cl 1. SOCl2 2. HOCH2CH2N(CH2CH3)2 • Why does the amino alcohol not react with the acyl halide at nitrogen in the last step? After all, amines are more nucleophilic than alcohols. The answer is: It does, but because the amine function is tertiary, it can only form an acyl ammonium salt. This function has reactivity similar to that of an acyl chloride. Thus, attack by the hydroxy group on the acyl ammonium carbonyl carbon wins out thermo- dynamically to give the ester in the end (Section 20-2). B HEC O R Clð NR�3ðš š B HEC O R NR3� Clð ð š� ðš š � � � B HEC O R OR��� NR�3 R��OH Exercise 22-20 Try It Yourself Device an alternative route to proparacaine from 4-hydroxybenzenecarboxylic acid, using Pd catalysis. 2 2 - 6 E l e c t r o p h i l i c S u b s t i t u t i o n o f P h e n o l s Under basic conditions, phenols can undergo electrophilic substitution, even with very mild electrophiles, through intermediate phenoxide ions. An industrially important applica- tion is the reaction with formaldehyde, which leads to o- and p-hydroxymethylation. Mech- anistically, these processes may be considered enolate condensations, much like the aldol reaction (Section 18-5). 1046 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s CH2 OH HO� �k � O � O O � � HOH PO Hydroxymethylation of Phenol ððš šš šð ðð ðð šš O H CH2O � � O H� OCH2 HOH �HO � OH CH2OH � OH CH2OHðš ðš ðððð ð š š šš šš š ðš šš ššš The initial aldol products are unstable: They dehydrate on heating, giving reactive interme- diates called quinomethanes. HO�, � �HOH OH CH2OH O CH2OH CH2 OH HO�, � �HOH O p-Quinomethane CH2 o-Quinomethane Because quinomethanes are a,b-unsaturated carbonyl compounds, they may undergo Michael additions (Section 18-11) with excess phenoxide ion. The resulting phenols can be hydroxymethylated again and the entire process repeated. Eventually, a complex phenol – formaldehyde copolymer, also called a phenolic resin (e.g., Bakelite), is formed. Their major uses are in plywood (45%), insulation (14%), molding compounds (9%), fi brous and granulated wood (9%), and laminates (8%). OH CH2 H O� CH2OH OH CH2 O CH2 O� CH2 � O O Phenolic Resin Synthesis O H CH2 OH OH H2C CH2 OH OH polymer n H2O� P� k C h a p t e r 2 2 1047 In the Kolbe*-Schmitt† reaction, phenoxide attacks carbon dioxide to furnish the salt of 2-hydroxybenzoic acid (o-hydroxybenzoic acid, salicylic acid, precursor to aspirin; see Chemical Highlight 22-2). C O �Na� OH C O NaOH, H2O, pressure � OH B O B ðð ðð O B ðð ðHšš *Professor Adolph Wilhelm Hermann Kolbe (1818–1884), University of Leipzig, Germany. †Professor Rudolf Schmitt (1830–1898), University of Dresden, Germany. Exercise 22-21 Working with the Concepts: Recognizing Phenol as an Enol Formulate a mechanism for the Kolbe-Schmitt reaction. Strategy As always, we take an inventory of the components of the reaction: starting materials, other reagents and reaction conditions, and products. Phenol is an electron-rich arene (see this and Section 16-3) and also acidic. Carbon dioxide has an electrophilic carbon that is attacked by nucleophilic carbon atoms, such as in Grignard reagents (Section 19-6). The reaction conditions are strongly basic. Finally, the product looks like that of an electrophilic ortho substitution. Solution • Under basic conditions, phenol will exist as phenoxide, which, like an enolate ion (Section 18-1), can be described by two resonance forms (see margin). • Enolate alkylations occur at carbon (Section 18-10). In analogy, you can formulate a phenoxide attack on the electrophilic carbon of CO2. Alternatively, you can think of this reaction as an elec- trophilic attack of CO2 on a highly activated benzene ring. • Finally, deprotonation occurs to regenerate the aromatic arene. O) ) O) ) O) ) p � O) p p � OH) p OH p p B B HC O) ) B C O) ) O H B E C As you will have noticed, the selectivity for ortho attack by CO2 in this process is exceptional. Although not completely understood, it may involve direction of the electrophile by the Na1 ion in the vicinity of the phenoxide negative charge. Exercise 22-22 Try It Yourself Phentolamine (as a water-soluble methanesulfonic acid salt) is an antihypertensive that has recently been introduced into dentistry: It cuts in half the time taken to recover from the numbing effect of local anesthetics. The key step in its preparation dates from 1886, the reaction shown below. What is its mechanism? (Caution: This is not a nucleophilic aromatic substitution. Hint: Think keto – enol tautomerism). � OH CH3 NH2 OH 70% Phentolamine 160�C CH3 OH N H N HN CH3 OH N Various wood products incorpo- rating Bakelite are used in the construction of houses. 2 2 - 6 E l e c t r o p h i l i c S u b s t i t u t i o n o f P h e n o l s Phenoxide ion O � ) ) O) ) p ½ � 1048 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s In Summary The benzene ring in phenols is subject to electrophilic aromatic substitu- tion, particularly under basic conditions. Phenoxide ions can be hydroxymethylated and carbonated. 22-7 An Electrocyclic Reaction of the Benzene Ring: The Claisen Rearrangement At 2008C, 2-propenyloxybenzene (allyl phenyl ether) undergoes an unusual reaction that leads to the rupture of the allylic ether bond: The starting material rearranges to 2-(2-propenyl)phenol (o-allylphenol). 2-Propenyloxybenzene (Allyl phenyl ether) 2-(2-Propenyl)phenol (o-Allylphenol) 75% H O CH2CH CH2E P CH2PCH2CH OH � This transformation, called the Claisen* rearrangement, is another concerted reaction with a transition state that accommodates the movement of six electrons (Sections 14-8 and 15-3). The initial intermediate is a high-energy isomer, 6-(2-propenyl)-2,4-cyclohexadienone, which enolizes to the fi nal product (Sections 18-2 and 22-3). Mechanism of the Claisen Rearrangement O OH H O 6-(2-Propenyl)- 2,4-cyclohexadienone O O O The Claisen rearrangement is general for other systems. With the nonaromatic 1- ethenyloxy- 2-propene (allyl vinyl ether), it stops at the carbonyl stage because there is no driving force for enolization. This is called the aliphatic Claisen rearrangement. Exercise 22-23 Hexachlorophene (margin) is a skin germicide formerly used in soaps. It is prepared in one step from 2,4,5-trichlorophenol and formaldehyde in the presence of sulfuric acid. How does this reac- tion proceed? (Hint: Formulate an acid-catalyzed hydroxymethylation for the fi rst step.) *Professor Ludwig Claisen (1851–1930), University of Berlin, Germany. REACTION MECHANISM Hexachlorophene Cl Cl OH OH Cl Cl Cl Cl C h a p t e r 2 2 1049 Aliphatic Claisen Rearrangement 1-Ethenyloxy-2-propene (Allyl vinyl ether) 50% 4-Pentenal 255°C H C C H NCH2 CH2 E KH H2C O A H C C H HCH2 CH2 K EN H2C O A The carbon analog of the Claisen rearrangement is called the Cope* rearrangement; it takes place in compounds containing 1,5-diene units. 178°C 3-Phenyl-1,5-hexadiene Cope Rearrangement trans-1-Phenyl-1,5-hexadiene 72% Note that all of these rearrangements are related to the electrocyclic reactions that inter- convert cis-1,3,5-hexatriene with 1,3-cyclohexadiene (see margin and Section 14-9). The only difference is the absence of a double bond connecting the terminal p bonds. 2 2 - 7 C l a i s e n R e a r r a n g e m e n t *Professor Arthur C. Cope (1909–1966), Massachusetts Institute of Technology, Cambridge. Exercise 22-24 Explain the following transformation by a mechanism. (Hint: The Cope rearrangement can be accelerated greatly if it leads to charge delocalization.) OHC HO NaOH, H2O Exercise 22-25 Working with the Concepts: Applying Claisen and Cope Rearrangements Citral, B, is a component of lemon grass and as such used in perfumery (lemon and verbena scents). It is also an important intermediate in the BASF synthesis of vitamin A (Section 14-7, Chemical Highlight 18-3). The last step in the synthesis of citral requires simply heating the enol ether A. How do you get from A to B? H O B Citral O A � Electrocyclic Reaction of cis-1,3,5-Hexatriene � Strategy As always, we take an inventory of the components of the reaction: starting materials, other reagents and reaction conditions, and products. Here, this is straightforward: There are no reagents, we simply apply heat, and it appears that the reaction is an isomerization. You need to confi rm this suspicion, by determining the molecular formulas of A and B. Indeed, it is C10H16O for both. What thermal reactions could you envisage for A? Solution • You note that A contains a diene unit connected to an isolated double bond. Hence, in principle, an intramolecular Diels-Alder reaction to C might be feasible (Section 14-8; Exercise 14-24): Potential Diels-Alder Reaction of A C O A O • It is apparent that this pathway is unfavorable for two reasons: (1) Both the diene and the dieno- phile are electron rich and therefore not good partners for cycloaddition (Section 14-8), and, even more obvious, (2) a strained ring is formed in C. • An alternative is based on the recognition of a 1,5-hexadiene unit, the prerequisite for a Cope rearrangement. In A, this diene unit contains an oxygen, hence we can write a Claisen rearrange- ment to see where it leads us. DA � H O Claisen Rearrangement of A O • The product D contains a 1,5-diene substructure, capable of a Cope rearrangement, leading to citral B. BD �O H H O Cope Rearrangement of D Exercise 22-26 Try It Yourself The ether A provides B upon heating to 2008C. Formulate a mechanism. (Caution: The terminal alkenyl carbon cannot reach the para position of the benzene ring. Hint: Start with the fi rst step of a Claisen rearrangement.) OH CH3H3C O CH3H3C BA � 1050 C h a p t e r 2 2 1051 In Summary 2-Propenyloxybenzene rearranges to 2-(2-propenyl)phenol (o-allylphenol) by an electrocyclic mechanism that moves six electrons (Claisen rearrangement). Similar con- certed reactions are undergone by aliphatic unsaturated ethers (aliphatic Claisen rearrange- ment) and by hydrocarbons containing 1,5-diene units (Cope rearrangement). 22-8 Oxidation of Phenols: Benzoquinones Phenols can be oxidized to carbonyl derivatives by one-electron transfer mechanisms, result- ing in a new class of cyclic diketones, called benzoquinones. Benzoquinones and benzenediols are redox couples The phenols 1,2- and 1,4-benzenediol (for which the respective common names cat echol and hydroquinone are retained by IUPAC) are oxidized to the corresponding diketones, ortho- and para-benzoquinone, by a variety of oxidizing agents, such as sodium dichro- mate or silver oxide. Yields can be variable when the resulting diones are reactive, as in the case of o-benzoquinone, which partly decomposes under the conditions of its formation. Benzoquinones from Oxidation of Benzenediols OH OH O O Low yield Ag2O, (CH3CH2)2O OH OH O 92% O Na2Cr2O7, H2SO4 Hydroquinone p-Benzoquinone O O O O Catechol o-Benzoquinone O O O O The redox process that interconverts hydroquinone and p-benzoquinone can be visual- ized as a sequence of proton and electron transfers. Initial deprotonation gives a phenoxide ion, which is transformed into a phenoxy radical by one-electron oxidation. Proton 2 2 - 8 O x i d a t i o n o f P h e n o l s : B e n z o q u i n o n e s 1052 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s dissociation from the remaining OH group furnishes a semiquinone radical anion, and a second one-electron oxidation step leads to the benzoquinone. All of the intermediate species in this sequence benefi t from considerable resonance stabilization (two forms are shown for the semiquinone). We shall see in Section 22-9 that redox processes similar to those shown here occur widely in nature. OH OH �H� OH O � �e� Phenoxide ion OH Phenoxy radical jO �H� jO Semiquinone radical anion �e� O OjO ð ð�� ðšðšðš ðš O �ððš O �ðð�ð ðð ðð Redox Relation Between p-Benzoquinone and Hydroquinone �ð �ð Exercise 22-27 Give a minimum of two additional resonance forms each for the phenoxide ion, phenoxy radical, and semiquinone radical anion shown in the preceding scheme. The enone units in p-benzoquinones undergo conjugate and Diels-Alder additions p-Benzoquinones function as reactive a,b-unsaturated ketones in conjugate additions (see Section 18-9). For example, hydrogen chloride adds to give an intermediate hydroxy dienone that enolizes to the aromatic 2-chloro-1,4-benzenediol. O OH p-Benzoquinone O O � HCl H Cl 6-Chloro-4-hydroxy- 2,4-cyclohexadienone OH OH Cl 2-Chloro-1,4-benzenediol The double bonds also undergo cycloadditions to dienes (Section 14-8). The initial cyclo- adduct to 1,3-butadiene tautomerizes with acid to the aromatic system. � OH OH 88% overall O O C6H6, 20�C, 48 h O O H % % H HCl, � Diels–Alder Reactions of p-Benzoquinone C h a p t e r 2 2 1053 In Summary Phenols are oxidized to the corresponding benzoquinones. The diones enter into reversible redox reactions that yield the corresponding diols. They also undergo conjugate additions and Diels-Alder additions to the double bonds. 22-9 Oxidation-Reduction Processes in Nature This section describes some chemical processes involving hydroquinones and p-benzoquinones that occur in nature. We begin with an introduction to the biochemical reduction of O2. Oxygen can engage in reactions that cause damage to biomolecules. Naturally occurring antioxidants inhibit these transformations, as do several synthetic preservatives. Ubiquinones mediate the biological reduction of oxygen to water Nature makes use of the benzoquinone – hydroquinone redox couple in reversible oxidation reactions. These processes are part of the complicated cascade by which oxygen is used in biochemical degradations. An important series of compounds used for this purpose are the ubiquinones (a name coined to indicate their ubiquitous presence in nature), also collectively called coenzyme Q (CoQ, or simply Q). The ubiquinones are substituted p-benzoquinone derivatives bearing a side chain made up of 2-methylbutadiene units (isoprene; Sections 4-7 and 14-10). An enzyme system that utilizes NADH (Chemical Highlights 8-1 and 25-2) converts CoQ into its reduced form (QH2). CHEMICAL HIGHLIGHT 22-3 Chemical Warfare in Nature: The Bombardier Beetle The oxidizing power of p-benzoquinones is used by some arthropods, such as millipedes, beetles, and termites, as chemical defense agents. Most remarkable among these species is the bombardier beetle. Its name is descriptive of its defense mechanism against predators, usually ants, which involves fi ring hot corrosive chemicals from glands in its posterior, with amazing accuracy. At the time of an attack (simulated in the laboratory by pinching the beetle with fi ne- tipped forceps, see photo), two glands located near the end of the beetle’s abdomen secrete mainly hydroquinone and hydrogen peroxide, respectively, into a reaction chamber. This chamber contains enzymes that trigger the explosive oxidation of the diol to the quinone and simultaneous decomposition of hydrogen peroxide to oxygen gas and water. This cocktail is audibly expelled at temperatures up to 1008C in the direction of the enemy from the end of the beetle’s abdomen, aided for aim by a 2708 rotational capability. In some species, fi ring occurs in pulses of about 500 per second, like a machine gun. The bombardier beetle in action. Exercise 22-28 Explain the following result by a mechanism. (Hint: Review Section 18-9.) CH3 OCH3 H3C CH3O O O CH3 OCH2CH3 H3C CH3CH2O O O CH3CH2O�Na�, CH3CH2OH 2 2 - 9 O x i d a t i o n - R e d u c t i o n P r o c e s s e s i n N a t u r e 1054 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s OH OH (CH2CH CCH2)nH CH3 CH3 CH3O CH3O O Ubiquinones (n � 6, 8, 10) (Coenzyme Q) O P A (CH2CH CCH2)nH CH3 CH3 CH3O CH3O Enzyme, reducing agent Reduced form of coenzyme Q (Reduced Q, or QH2) P A QH2 participates in a chain of redox reactions with electron-transporting iron-containing proteins called cytochromes (Chemical Highlight 8-1). The reduction of Fe31 to Fe21 in cyto- chrome b by QH2 begins a sequence of electron transfers involving six different proteins. The chain ends with reduction of O2 to water by addition of four electrons and four protons. O2 1 4 H 1 1 4 e2 uy 2 H2O Phenol derivatives protect cell membranes from oxidative damage The biochemical conversion of oxygen into water includes several intermediates, including superoxide, O2 . 2, the product of one-electron reduction, and hydroxy radical, . OH, which arises from cleavage of H2O2. Both are highly reactive species capable of initiating reactions that damage organic molecules of biological importance. An example is the phosphogly- ceride shown here, a cell-membrane component derived from the unsaturated fatty acid cis,cis-octadeca-9,12-dienoic acid (linoleic acid). Initiation step R R� CH2(CH2)6CO CH2O C(CH2)14CH3 CH O O B O R� O B B A A A O CH2O PO(CH2)2N(CH3)3 O� O OH �HOH j j R R� j R Pentadienyl radical R� j C H HCH3(CH2)4 11 913 � The doubly allylic hydrogens at C11 are readily abstracted by radicals such as . OH (Section 14-2). The resonance-stabilized pentadienyl radical combines rapidly with O2 in the fi rst of two propagation steps. Reaction occurs at either C9 or C13 (shown here), giving either of two peroxy radicals containing conjugated diene units. Propagation step 1 R Peroxy radical R� j R R�O2 A jOð� Oðš C h a p t e r 2 2 1055 In the second propagation step this species removes a hydrogen atom from C11 of another molecule of phosphoglyceride, or more generally, lipid (Section 20-5), thereby forming a new dienyl radical and a molecule of lipid hydroperoxide. The dienyl radical may then reenter propagation step 1. In this way, a large number of lipid molecules may be oxidized following just a single initiation event. Propagation step 2 � � Lipid hydroperoxide R RR� R� j O OH R R� R R� A C H 11 H A jOð� Oðš ð� ðš Numerous studies have confi rmed that lipid hydroperoxides are toxic, their products of decomposition even more so. For example, loss of .OH by cleavage of the relatively weak O – O bond gives rise to an alkoxy radical, which may decompose by breaking a neighboring C – C bond (b-scission), forming an unsaturated aldehyde. R O R�H R�H O R O H OH � jOH R -Scission of a Lipid Alkoxy Radical� Alkoxy radical � jR�Oj ð� ðš � � � � ð ð Through related but more complex mechanisms, certain lipid hydroperoxides decom- pose to give unsaturated hydroxyaldehydes, such as trans-4-hydroxy-2-nonenal, as well as the dialdehyde propanedial (malondialdehyde). Molecules of these general types are partly responsible for the smell of rancid fats. Both propanedial and the a,b-unsaturated aldehydes are extremely toxic, because they are highly reactive toward the proteins that are present in close proximity to the lipids in cell membranes. For example, both dials and enals are capable of reacting with nucleo- philic amino and mercapto groups from two different parts of one protein or from two different protein molecules, and these reactions produce cross-linking (Section 14-10). Cross-linking severely inhibits protein molecules from carrying out their biological func- tions (Chapter 26). R H Cross-linking of Proteins by Reaction with Unsaturated Aldehydes O H Protein1OSH protein1 SO R O Protein2ONH2 H protein1 SO R N protein2O Processes such as these are thought by many to contribute to the development of emphy- sema, atherosclerosis (the underlying cause of several forms of heart disease and stroke), certain chronic infl ammatory and autoimmune diseases, cancer, and, possibly, the process of aging itself. Does nature provide the means for biological systems to protect themselves from such damage? A variety of naturally occurring antioxidant systems defend lipid molecules inside cell membranes from oxidative destruction. The most important is vitamin E, a collection trans-4-Hydroxy-2-nonenal CH3(CH2)4 O H HO Propanedial (Malondialdehyde) H O O H 2 2 - 9 O x i d a t i o n - R e d u c t i o n P r o c e s s e s i n N a t u r e 1056 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s of eight compounds with very similar structures, commonly represented by one of them, a-tocopherol (margin). They all possess a long hydrocarbon chain (see Problem 46 of Chapter 2), a feature that makes them lipid soluble. Their reducing qualities stem from the presence of the hydroquinone-like aromatic ring (Section 22-8). The corresponding phenoxide ion is an excellent electron donor. The protective qualities of vitamin E rest on its ability to break the propagation chain of lipid oxidation by the reduction of radical species. O CH3 Vitamin E H3C HO CH3 Reactions of Vitamin E with Lipid Hydroperoxy and Alkoxy Radicals �H� � O CH3 lipid O e-Transfer or Lipid radicalsVitamin E phenoxide � O j lipid OO OO j �H� O CH3 � H3C O CH3 lipid or -Tocopheroxy radical � O lipid OO O lipid OH or O lipid OO OHO j CH3 R CH3 R � O �ð š �š �š �š �š �š�š ð�š O �ð�š �š �š �š O CH3 H3C CH3 R In this process, lipid radicals are reduced and protonated. Vitamin E is oxidized to an a-tocopheroxy radical, which is relatively unreactive because of extensive delocal- ization and the steric hindrance of the methyl substituents. Vitamin E is regenerated at the membrane surface by reaction with water-soluble reducing agents such as vitamin C. � � CH2OH Vitamin C OH O O HHO H# CH2OH Semidehydroascorbic acid OH O O HHO H# O CH3 H3C CH3 Regeneration of Vitamin E by Vitamin C O CH3 H3C CH3 CH3 R CH3 R Oj�š Oj�š� Oð�š � Oð�š The product of vitamin C oxidation eventually decomposes to lower-molecular-weight water- soluble compounds, which are excreted by the body. Lipid peroxidation has been implicated in retinal diseases, and clinicians prescribe antioxidants to help in the treatment of diabetic retinopathy. This condition is characterized by a compromised blood supply that causes the development of fatty exudates and fi brous tissue on the retina (yellow areas in photo of the eye). Exercise 22-29 Vitamin C is an effective antioxidant because its oxidation product semidehydroascorbic acid is stabilized by resonance. Give other resonance forms for this species. HO CH3 O CH3 R H3C CH3 Vitamin E ( -Tocopherol)� R � branched C16H33 chain C h a p t e r 2 2 1057 Benzoquinones consume glutathione, an intracellular reducing agent Virtually all living cells contain the substance glutathione, a peptide that incorporates a mercapto functional group (see Sections 9-10 and 26-4). The mercapto function serves to reduce disulfi de linkages in proteins to SH groups and to maintain the iron in hemo- globin in the 21 oxidation state (Section 26-8). Glutathione also participates in the redu- ction of oxidants, such as hydrogen peroxide, H2O2, that may be present in the interior of the cell. H3NCHCH2CH2 HSCH2 Glutathione NHCH2COOH COO� C NHCH O � A AB O B O O CO O The molecule is converted into a disulfi de (Section 9-10) in this process but is regenerated by an enzyme-mediated reduction. 2 Glutathione glutathione glutathioneSH H2O2 2 H2O� �O SO SO O Glutathione peroxidase Glutathione reductases Benzoquinones and related compounds react irreversibly with glutathione in the liver by conjugate addition. Cell death results if glutathione depletion is extensive. Acetamino- phen is an example of a substance that exhibits such liver toxicity at very high doses. Cytochrome P-450, a redox enzyme system in the liver, oxidizes acetaminophen to an imine derivative of a benzoquinone, which in turn consumes glutathione. Vitamin C is capable of reversing the oxidation. S glutathione HNCCH3 O OH Cytochrome P-450 Vitamin C B HNCCH3 O OH B NCCH3 O Glutathione–SH O B O Acetaminophen Synthetic analogs of vitamin E are preservatives Synthetic phenol derivatives are widely used as antioxidants and preservatives in the food industry. Perhaps two of the most familiar are 2-(1,1-dimethylethyl)-4-methoxyphenol (butylated hydroxyanisole, or BHA) and 2,6-bis(1,1-dimethylethyl)-4-methylphenol (butylated hydroxytoluene, or BHT; see Exercise 16-14). For example, addition of BHA to butter increases its storage life from months to years. Both BHA and BHT function like vitamin E, reducing oxygen radicals and interrupting the propagation of oxidation processes. In Summary Oxygen-derived radicals are capable of initiating radical chain reactions in lipids, thereby leading to toxic decomposition products. Vitamin E is a naturally occurring phenol derivative that functions as an antioxidant to inhibit these processes within mem- brane lipids. Vitamin C and glutathione are biological reducing agents located in the intra- and extracellular aqueous environments. High concentrations of benzoquinones can bring C(CH3)3 OCH3 2-(1,1-Dimethylethyl)- 4-methoxyphenol (BHA) OH C(CH3)3(CH3)3C CH3 2,6-Bis(1,1-dimethylethyl)- 4-methylphenol (BHT) OH 2 2 - 9 O x i d a t i o n - R e d u c t i o n P r o c e s s e s i n N a t u r e 1058 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s about cell death by consumption of glutathione; vitamin C can protect the cell by reduction of the benzoquinone. Synthetic food preservatives are structurally designed to mimic the antioxidant behavior of vitamin E. 22-10 Arenediazonium Salts As mentioned in Section 22-4, N-nitrosation of primary benzenamines (anilines) furnishes arenediazonium salts, which can be used in the synthesis of phenols. Arenediazonium salts are stabilized by resonance of the p electrons in the diazo function with those of the aromatic ring. They are converted into haloarenes, arenecarbonitriles, and other aromatic derivatives through replacement of nitrogen by the appropriate nucleophile. Arenediazonium salts are stabilized by resonance The reason for the stability of arenediazonium salts, relative to their alkane counterparts, is resonance and the high energy of the aryl cations formed by loss of nitrogen. One of the electron pairs making up the aromatic p system can be delocalized into the functional group, which results in charge-separated resonance structures containing a double bond between the benzene ring and the attached nitrogen. N N ðš ð N� N N� N � B � ðð N� N B � � ðð N� N � B � Resonance in the Benzenediazonium Cation At elevated temperatures (.508C), nitrogen extrusion does take place, however, to form the very reactive phenyl cation. When this is done in aqueous solution, phenols are produced (Section 22-4). Why is the phenyl cation so reactive? After all, it is a carbocation that is part of a benzene ring. Should it not be resonance stabilized, like the phenylmethyl (benzyl) cation? The answer is no, as may be seen in the molecular-orbital picture of the phenyl cation (Figure 22-5). The empty orbital associated with the positive charge is one of the sp2 hybrids aligned in perpendicular fashion to the p framework that normally produces aromatic res- onance stabilization. Hence, this orbital cannot overlap with the p bonds, and the positive + p orbitals Empty sp2 orbital BA C + Phenyl cation Figure 22-5 (A) Line structure of the phenyl cation. (B) Orbital picture of the phenyl cation. The alignment of its empty sp2 orbital is perpendicular to the six-p-electron framework of the aromatic ring. As a result, the positive charge is not stabilized by resonance. (C) The electrostatic potential map of the phenyl cation, shown at an attenuated scale for better contrast, pinpoints much of the positive charge (blue edge on the right) as located in the plane of the six-membered ring. C h a p t e r 2 2 1059 charge cannot be delocalized. Moreover, the cationic carbon would prefer sp hybridization, an arrangement precluded by the rigid frame of the benzene ring. We used a similar argu- ment to explain the diffi culty in deprotonating benzene to the corresponding phenyl anion (Section 22-4). Arenediazonium salts can be converted into other substituted benzenes When arenediazonium salts are decomposed in the presence of nucleophiles other than water, the corresponding substituted benzenes are formed. For example, diazotization of arenamines (anilines) in the presence of hydrogen iodide results in the corresponding iodoarenes. CHO NH2 N2 OO O CHO � I OO O 53% CH3COOH, HI, NaNO2 Attempts to obtain other haloarenes in this way are frequently complicated by side reac- tions. One solution to this problem is the Sandmeyer* reaction, which makes use of the fact that the exchange of the nitrogen substituent for halogen is considerably facilitated by the presence of cuprous [Cu(I)] salts. The detailed mechanism of this process is complex, and radicals are participants. Addition of cuprous cyanide, CuCN, to the diazonium salt in the presence of excess potassium cyanide gives aromatic nitriles. Sandmeyer Reactions Cl NH2 NH2 2-Methylbenzenamine (o-Methylaniline) 2-Chlorobenzenamine (o-Chloroaniline) Cl Br 1-Bromo-2-chlorobenzene (o-Bromochlorobenzene) 73% CH3 Cl 1-Chloro-2-methylbenzene (o-Chlorotoluene) 79% overall CH3 �N2 N2�Cl� CH3 HCl, NaNO2, 0°C 1. HBr, NaNO2, 0°C 2. CuBr, 100°C CuCl, 60°C NH2 CH3 CN 70% 4-Methylbenzonitrile ( p-Tolunitrile) CH3 1. HCl, NaNO2, 0°C 2. CuCN, KCN, 50°C *Dr. Traugott Sandmeyer (1854 – 1922), Geigy Company, Basel, Switzerland. 2 2 - 1 0 A r e n e d i a z o n i u m S a l t s 1060 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s The diazonium group can be removed by reducing agents. The sequence diazotization – reduction is a way to replace the amino group in arenamines (anilines) with hydrogen. The reducing agent employed is aqueous hypophosphorous acid, H3PO2. This method is especially useful in syntheses in which an amino group is used as a removable directing substituent in electrophilic aromatic substitution (Section 16-5). N2� NaNO2, H�, H2O 1-Bromo-3-methylbenzene (m-Bromotoluene) 85% Reductive Removal of a Diazonium Group NH2 CH3 Br CH3 Br H3PO2, H2O, 25�C CH3 Br H Another application of diazotization in synthetic strategy is illustrated in the synthesis of 1,3-dibromobenzene (m-dibromobenzene). Direct electrophilic bromination of benzene is not feasible for this purpose; after the fi rst bromine has been introduced, the second will attack ortho or para. What is required is a meta-directing substituent that can be transformed eventually into bromine. The nitro group is such a substituent. Double nitration of benzene furnishes 1,3-dinitrobenzene (m-dinitrobenzene). Reduction (Section 16-5) leads to the ben- zenediamine, which is then converted into the dihalo derivative. HNO3, H2SO4, � Synthesis of 1,3-Dibromobenzene by Using a Diazotization Strategy NO2 H2, Pd Br BrNO2 NH2 NH2 1. NaNO2, H�, H2O 2. CuBr, 100�C Exercise 22-31 Propose a synthesis of 1,3,5-tribromobenzene from benzene. In Summary Arenediazonium salts, which are more stable than alkanediazonium salts because of resonance, are starting materials not only for phenols, but also for haloarenes, Exercise 22-30 Propose syntheses of the following compounds, starting from benzene. (a) I (b) CN CN (c) SO3H OH C h a p t e r 2 2 1061 arenecarbonitriles, and reduced aromatics by displacement of nitrogen gas. The intermediates in some of these reactions may be aryl cations, highly reactive because of the absence of any electronically stabilizing features, but other, more complicated mechanisms may be followed. The ability to transform arenediazonium salts in this way gives considerable scope to the regioselective construction of substituted benzenes. 22-11 Electrophilic Substitution with Arenediazonium Salts: Diazo Coupling Being positively charged, arenediazonium ions are electrophilic. Although they are not very reactive in this capacity, they can accomplish electrophilic aromatic substitution when the substrate is an activated arene, such as phenol or benzenamine (aniline). This reaction, called diazo coupling, leads to highly colored compounds called azo dyes. For example, reaction of N,N-dimethylbenzenamine (N,N-dimethylaniline) with benzenediazonium chloride gives the brilliant orange dye Butter Yellow. This compound was once used as a food coloring agent but has been declared a suspect carcinogen by the Food and Drug Administration. Diazo Coupling N(CH3)2 Cl� � N� � Cl� )N(CH3)2 H�, H2O pH 3–5 J � � J � � N(CH3)2 �HCl Azo function 4-Dimethylaminoazobenzene (p-Dimethylaminoazobenzene, Butter Yellow) Dyes used in the clothing industry usually contain sulfonic acid groups that impart water solubility and allow the dye molecule to attach itself ionically to charged sites on the poly- mer framework of the textile. pH � 3.0, blue-violet pH � 5.0, red NH2 H2N N N N N Na��O3S SO3�Na� P P Congo Red J � � SO3�Na� Methyl Orange (CH3)2N Industrial Dyes pH � 3.1, red pH � 4.4, yellow N N O Na S O O Dyes are important additives in the textile industry. Azo dyes, while still in widespread use, are becoming less attractive for this purpose because some have been found to degrade to carcinogenic benzenamines. 2 2 - 1 1 E l e c t r o p h i l i c S u b s t i t u t i o n w i t h A r e n e d i a z o n i u m S a l t s 1062 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s CHEMICAL HIGHLIGHT 22-4 William Perkin and the Origins of Industrial and Medicinal Chemistry In 1851, William Perkin* was a 13-year-old pupil at City of London School. His teacher, Thomas Hall, recognized Perkin’s interest and aptitude in science but also realized his own limitations. Hall therefore encouraged Perkin to attend lectures at the Royal Institution, including a series presented by the legendary scientist Michael Faraday.† A few years earlier, Faraday and others had sponsored the founding of the Royal College of Chemistry in response to Justus Liebig’s‡ scathing criticism of the sorry state of chemical research in England. Liebig’s brilliant young student, August von Hofmann,§ was hired as its fi rst director. London in the 1850s was perhaps the fi lthiest, most polluted city in the world. It was lit by gas lamps, with the gas extracted from coal. The residue of the extraction was massive quantities of coal tar, most of which was simply dumped into London’s streams and rivers. However, coal tar was rich in aromatic amines, and Hofmann, who found these compounds fascinating, could obtain coal tar in London by the barrelful, just by asking. By 1856, Perkin had begun studying in Hofmann’s labo- ratory. Malaria was rampant in England (indeed, in most of the world) at the time, and the only remedy was quinine, the single source of which was the bark of the South American cinchona tree. Perkin knew that the molecular formula of quinine was C20H24N2O2 and that Hofmann had isolated several 10-carbon amines, including naphthalenamine, C10H9N. Lacking any knowledge of molecular structure, Perkin decided to try to synthesize quinine by oxidative dimerization of such coal tar – derived 10-carbon aromatic amines. What could be more useful than to convert a major environmental pollutant into a desperately needed medicine? Perkin’s experiments gave black crude products, out of which could be extracted brightly colored powders. Pursuing these experiments with more amines and, eventually, amine mixtures, Perkin stumbled onto a procedure that gave a brilliant purple substance to which was given the name “mauve.” Within a short time, dyes based on Perkin’s mauve had transformed the world of fashion — not exactly what he had in mind originally, but the discovery did make him very rich. Previously, the only source of purple dye (so-called Royal Purple, because only royalty could afford it) was from the shell lining of a species of mollusk found in only two locations on the Mediterranean coast. Thus was born the synthetic dye industry, the fi rst of the large-scale industries based on chemical science. Remarkably, the correct basic chemical structure of Perkin’s mauve was not determined and published until 1994, by Professor Otto Meth-Cohn and undergraduate Mandy Smith, at the University of Sunderland, England. The reaction that forms the major constituent of the dye is shown below. *Sir William Henry Perkin (1838 – 1907), Royal College of Chemistry, London, England. †This the same Faraday as the discoverer of benzene (Chapter 15 Opening). ‡Baron Justus von Liebig (1803 – 1873), University of Munich, Germany. §This is the same Hofmann as that of the Hofmann rule (Section 11-6) and the Hofmann rearrangement (Section 20-7). 2 NH2 K2Cr2O7, H2SO4, H2O CH3 H3C � � N N �NH2 CH3 NH2 N H CH3 NH2 In Summary Arenediazonium cations attack activated benzene rings by diazo coupling, a process that furnishes azobenzenes, which are often highly colored. Exercise 22-32 Write the products of diazo coupling of benzenediazonium chloride with each of the following molecules. (a) Methoxybenzene; (b) 1-chloro-3-methoxybenzene; (c) 1-(dimethylamino)-4- (1,1-dimethylethyl)benzene. (Hint: Diazo couplings are quite sensitive to steric effects.) C h a p t e r 2 2 1063 The process involves oxidation of some of the amine starting material to nitrosobenzenes, such as C6H5NO, followed by acid-catalyzed electrophilic aromatic substitution reactions between the nitroso compounds and other molecules of amine. of bacteriology, began to follow up on reports that anthrax was caused by microscopic rod-shaped bodies found in the blood of infected sheep. He developed practical techniques for culturing and growing colonies of bacteria, a concept introduced by Pasteur a generation earlier but never demon- strated. Koch went on to discover the microorganisms that caused tuberculosis, conjunctivitis, amebic dysentery, and cholera, publishing superb micrographs to support his fi nd- ings. By the last decades of the nineteenth century, Paul Ehrlich** became aware of reports that Perkin’s dyes could be used to stain cells, and he applied these ideas to help better visualize Koch’s tuberculosis bacilli. He realized that the staining phenomena were in fact chemical reactions and that the dyes stained different microbiological entities differ- ently. Ehrlich found that some dyes could be used as thera- peutic agents to combat the effects of bacterial toxins, thus pioneering the fi eld of chemotherapy and making great strides in the development of immunology. In 1910, Ehrlich developed the fi rst effective drug to combat syphilis, a dis- ease that had become a worldwide scourge. His work helped give birth to the fi elds of biochemistry, cell biology, and medicinal chemistry. Thus, Perkin’s discoveries of a half- century earlier had changed more than the world of fashion: They changed the world. Linda Evangelista models a lacy mauve dress from Christian Dior’s autumn collection: lmax 5 550 nm. ¶Professor H. H. Robert Koch (1843 – 1910), University of Göttingen, Germany, Nobel Prize 1905 (physiology or medicine). **Professor Paul Ehrlich, (1854 – 1915), Royal Institute for Experimental Therapy, Frankfurt, Germany, Nobel Prize 1908 (physiology or medicine). Scanning electron micrograph of Mycobacterium smegmatis found in soil. The species is used as a model for studies of tuberculosis caused by Mycobacterium tuberculosis. Image width: 8.38 3 1026 m. Perkin’s discovery did not help cure malaria, but it nevertheless had an enormous impact on medical science. In the 1860s, Robert Koch¶, one of the founders of the fi eld THE BIG PICTURE This chapter completes our understanding of the interplay between the benzene ring and its alkyl, hydroxy, and amino or modifi ed amino substituents. Just as much as the electronic character of such substituents may activate or deactivate the ring with respect to substitution (Chapter 16; Sections 22-4 and 22-6) and the way in which substitutents are positioned controls the location of attack by electrophiles and nucleophiles, the ring imparts special reactivity on attached nuclei because of its resonating capacity. In other words, and to repeat the underlying theme of the text, the structure of the substituted aromatic compound T h e B i g P i c t u r e 1064 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s determines its function. In many respects, this behavior is a simple extension of the chemistry of delocalized systems (Chapter 14) with the added feature of aromaticity (Chapter 15). In the next chapter, we shall continue to examine the effect that two functional groups in the same molecule have on each other, now turning to carbonyl groups. In Chapter 24, we shall study molecules containing the carbonyl and several hydroxy groups and their biological relevance. Then, in Chapters 25 and 26, we conclude with biologically important compounds containing an array of functionality. CHAPTER INTEGRATION PROBLEMS 22-33. 5-Amino-2,4-dihydroxybenzoic acid A is a potential intermediate in the preparation of natural products of medicinal value (Section 22-3). Propose syntheses, starting from methylbenzene (toluene). OH HO NH2 CH3 COOH A SOLUTION This problem builds on the expertise that you gained (Chapter 16) in controlling the substitution patterns of target benzenes, but now with a greatly expanded range of reactions. The key is, again, recognition of the directing power of substituents — ortho, para or meta (Section 16-2) — and their interconversion (Section 16-5). Retrosynthetic analysis of compound A reveals one carbon-based substituent, the carboxy group, which can be readily envisaged to be derivable from the methyl group in the starting material (by oxidation, Section 22-2). In the starting material, the carbon-based substituent is ortho, para directing, suggesting its use (retrosynthesis 1) in the introduction of the two hydroxy functions (as in compound B, through nitration – reduction – diazotization – hydrolysis; Sections 22-4 and 22-10). In compound A, it is meta directing and potentially utilizable (retrosynthesis 2) for the amination at C3 (as in compound C, through nitration – reduction). Retrosynthesis 1 HO OH COOH O2N NO2 COOH O2N NO2 CH3 CH3 A B E� Retrosynthesis 2 NO2 COOH NH2 COOH COOH CH3 A C E� The question is, Are compounds B and C effective precursors of compound A? The answer is yes. Nitration of compound B should take place at the desired position (C3 in the product), ortho and para, respectively, to the two hydroxy substituents, thus placing the nitrogen at its position in compound A. Electrophilic attack between the OH groups would be expected to be sterically hindered (Section 16-5). C h a p t e r 2 2 1065 Conversely, the amino group in compound C, especially when protected as an amide, should direct electrophilic substitution to the less hindered ortho carbon and the para carbon, again yielding the desired pattern. The actual proposed synthetic schemes would be then as follows. Synthesis 1 H2N NH2 COOH HO OH COOH O2N NO2 COOH O2N NO2 CH3CH3 Na2Cr2O7, H�, H2O 1. HNO3, H2SO4 2. H2, Ni 1. NaNO2, HCl 2. H2O, � H2, NiHNO3, H2SO4 A Synthesis 2 O2N NO2 COOH NO2 COOHCOOHCH3 NH2 COOH Na2Cr2O7, H�, H2O 1. H2, Ni 2. NaNO2, HCl 3. H2O, � 4. H�, H2O 1. CH3CCl 2. HNO3, H2SO4 H2, NiHNO3, H2SO4 AB B NCCH3 H O O 22-34. A key reaction in the early development of the “pill” (Section 4-7; Chemical Highlight 4-3) was the “dienone-phenol” rearrangement shown below. Write a mechanism. CH3 H3C H H O OH H� % % % C ≥ H ≥ CH3 CH3 H H HO OH % % C ≥ H ≥ SOLUTION This is a mechanistic problem involving an acid-catalyzed rearrangement featuring a migrating alkyl (methyl) group. Sound familiar? Review Section 9-3 on carbocation rearrangements! How do we obtain a suitable carbocation from our starting material? Answer: Protonation of the carbonyl group, which furnishes a resonance-stabilized hydroxypentadienyl cation (Section 14-6, 14-7, and Exercise 18-22). Protonation of Dienone Function H� CH3 % CH3 O % HO etc. š�ð� CH3 � � HO % � A C h a p t e r I n t e g r a t i o n P r o b l e m s 1066 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s Two of the resonance structures (write all of them) place a positive charge next to the methyl group. Only form A is “productive,” inasmuch as it leads to product by aromatization, which is the major driving force for the whole process. Methyl Shift and Phenol Formation �H� CH3 % HOš� � � H3C H% HOš� CH3 CH3 H H HO OH % % C C ≥ H ≥ A New Reactions Benzylic Resonance 1. Radical Halogenation (Section 22-1) RCHX RCHj j RCHRCH2 X2 � HX through etc. Benzylic radical Requires heat, light, or a radical initiator 2. Solvolysis (Section 22-1) RCHOR� RCH RCHRCHOSO2R SN1 � RSO3H� R�OH through � � etc. Benzylic cation 3. SN2 Reactions of (Halomethyl)benzenes (Section 22-1) CH2X CH2Nu � )Nu� � X� Through delocalized transition state 4. Benzylic Deprotonation (Section 22-1) CH3 CH2Li � RLi � RH Phenylmethyllithium (Benzyllithium)pKa 41 Oxidation and Reduction Reactions on Aromatic Side Chains 5. Oxidation (Section 22-2) RCH2 COOH 1. KMnO4, HO�, � 2. H�, H2OCrO3 OR C h a p t e r 2 2 1067 Benzylic alcohols RCHOH MnO2, acetone OR 6. Reduction by Hydrogenolysis (Section 22-2) CH2OR CH3 � ROHH2, Pd–C, ethanol C6H5CH2 is a protecting group for ROH. Phenols and Ipso Substitution 7. Acidity (Section 22-3) OH O O � � etc. Phenoxide ion ) ) p p � H� Much stronger acid than simple alkanols pKa 10 8. Nucleophilic Aromatic Substitution (Section 22-4) Nu � Cl� NO2 NO2 NuCl NO2 NO2 Cl NO2 NO2 Nucleophile attacks at ipso position. Nu)� � 9. Aromatic Substitution Through Benzyne Intermediates (Section 22-4) NaNH2, liquid NH3 Nucleophile attacks at both ipso and ortho positions. � NaCl NH3 NH2Cl OHX 1. NaOH, � 2. H�, H2O 10. Arenediazonium Salt Hydrolysis (Section 22-4) NaNO2, H�, 0�C Benzenediazonium cation NH2 OH N2� N2� H2O, � N e w R e a c t i o n s 1068 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 11. Substitution with Pd Catalysis KOH, Pd catalyst, PR3, 100�C X OH Reactions of Phenols and Alkoxybenzenes 12. Ether Cleavage (Section 22-5) Aryl C–O bond is not cleaved. OHOR RBr� HBr, � 13. Ether Formation (Section 22-5) Alkoxybenzene OR RX� NaOH, H2O Williamson method (Section 9-6) OH 14. Esterifi cation (Section 22-5) Phenyl alkanoate OCR RCCl O � Base OH B O B 15. Electrophilic Aromatic Substitution (Section 22-6) E OCH3 E � H� OCH3OCH3 � E� � 16. Phenolic Resins (Section 22-6) NaOH HO� OH CH2 O� � H2O OH CH2OH , NaOH OH OH polymer OH O CH2 P C h a p t e r 2 2 1069 17. Kolbe Reaction (Section 22-6) OH CO2� OH COOH1. NaOH, pressure 2. H�, H2O 18. Claisen Rearrangement (Section 22-7) Aromatic Claisen rearrangement O H O � OH Aliphatic Claisen rearrangement O H O 19. Cope Rearrangement (Section 22-7) R R 20. Oxidation (Section 22-8) 2,5-Cyclohexadiene-1,4-dione (p-Benzoquinone) O O OH OH Na2Cr2O7, H� 21. Conjugate Additions to 2,5-Cyclohexadiene-1,4-diones (p-Benzoquinones) (Section 22-8) O O � Cl OH HCl OH N e w R e a c t i o n s 1070 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 22. Diels-Alder Cycloadditions to 2,5-Cyclohexadiene-1,4-diones (p-Benzoquinones) (Section 22-8) O O O H H O � 0 % 23. Lipid Peroxidation (Section 22-9) O2, radical chain reaction followed by Fragmentation to alkoxy radicals, R R� C HH R R� C OOHH �-scission toxic substances such as 4-hydroxy-2-alkenals 24. Inhibition by Antioxidants (Section 22-9) Vitamin E (or BHA or BHT) HO CH3 CH3 H3C CH3 j R or � O O CH3 CH3 H3C CH3 O R � OOjš� Ojš� š�R R� C H R R� C H or OOH OH R R� C H R R� C H 25. Vitamin C as an Antioxidant (Section 22-9) �e� # CH2OH H O H O HO O OH � # CH2OH Semidehydroascorbic acid H Oj H O HO O OH š�ð š� Arenediazonium Salts 26. Sandmeyer Reactions (Section 22-10) CuX, � X Cl, Br, I, CN XN2�X� N2� 27. Reduction (Section 22-10) H3PO2 HN2� N2� C h a p t e r 2 2 1071 28. Diazo Coupling (Section 22-11) Azo compound Occurs only with strongly activated rings c N� H� N OH OH � � J � � Important Concepts 1. Phenylmethyl and other benzylic radicals, cations, and anions are reactive intermediates stabi- lized by resonance of the resulting centers with a benzene p system. 2. Nucleophilic aromatic ipso substitution accelerates with the nucleophilicity of the attacking species and with the number of electron-withdrawing groups on the ring, particularly if they are located ortho or para to the point of attack. 3. Benzyne is destabilized by the strain of the two distorted carbons forming the triple bond. 4. Phenols are aromatic enols, undergoing reactions typical of the hydroxy group and the aro- matic ring. 5. Benzoquinones and benzenediols function as redox couples in the laboratory and in nature. 6. Vitamin E and the highly substituted phenol derivatives BHA and BHT function as inhibitors of the radical-chain oxidation of lipids. Vitamin C also is an antioxidant, capable of regenerating vitamin E at the surface of cell membranes. 7. Arenediazonium ions furnish reactive aryl cations whose positive charge cannot be delocalized into the aromatic ring. 8. The amino group can be used to direct electrophilic aromatic substitution, after which it is replace- able by diazotization and substitution, including reduction. Problems 35. Give the expected major product(s) of each of the following reactions. (a) CH2CH3 Cl2(1 equivalent), hv (b) NBS (1 equivalent), hv 36. Formulate a mechanism for the reaction described in Problem 35(b). 37. Propose syntheses of each of the following compounds, beginning in each case with ethylbenzene. (a) (1-Chloroethyl)benzene; (b) 2-phenylpropanoic acid; (c) 2-phenylethanol; (d) 2-phenyloxacyclopropane. 38. Predict the order of relative stability of the three benzylic cations derived from chloromethyl- benzene (benzyl chloride), 1-(chloromethyl)-4-methoxybenzene (4-methoxybenzyl chloride), and 1-(chloromethyl)-4-nitrobenzene (4-nitrobenzyl chloride). Rationalize your answer with the help of resonance structures. 39. By drawing appropriate resonance structures, illustrate why halogen atom attachment at the para position of phenylmethyl (benzyl) radical is unfavored compared with attachment at the benzylic position. 40. Triphenylmethyl radical, (C6H5)3C?, is stable at room temperature in dilute solution in an inert solvent, and salts of triphenylmethyl cation, (C6H5)3C 1, can be isolated as stable crystalline solids. Propose explanations for the unusual stabilities of these species. P r o b l e m s 1072 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 41. Give the expected products of the following reactions or reaction sequences. (a) H2O, �BrCH2CH2CH2 CH2Br (b) 1. KCN, DMSO 2. H�, H2O, � CH2Cl (c) 1. CH3CH2CH2CH2Li, (CH3)2NCH2CH2N(CH3)2, THF 2. C6H5CHO 3. H�, H2O, � C16H14 42. The hydrocarbon with the common name fl uorene is acidic enough (pKa < 23) to be a useful indicator in deprotonation reactions of compounds of greater acidity. Indicate the most acidic hydrogen(s) in fl uorene. Draw resonance structures to explain the relative stability of its conju- gate base. 43. Outline a straightforward, practical, and effi cient synthesis of each of the following compounds. Start with benzene or methylbenzene. Assume that the para isomer (but not the ortho isomer) may be separated effi ciently from any mixtures of ortho and para substitution products. FluoreneFluorene (a) CH2CH2Br (b) CONH2 Cl (c) O COOCH3 (d) COOH BrBr 44. Rank the following compounds in descending order of reactivity toward hydroxide ion. Br NO2 Br Br NO2 NO2 Br NO2 NO2 Br NO2 NO2 45. Predict the main product(s) of the following reactions. In each case, describe the mechanism(s) in operation. (a) Cl NO2 NO2 H2NNH2 (b) Cl Cl O2N NO2 NaOCH3, CH3OH (c) Cl CH3 LiN(CH2CH3)2, (CH3CH2)2NH 46. Starting with benzenamine, propose a synthesis of aklomide, an agent used to treat certain exotic fungal and protozoal infections in veterinary medicine. Several intermediates are shown to give you the general route. Fill in the blanks that remain; each requires as many as three sequential reactions. (Hint: Review the oxidation of amino- to nitroarenes in Section 16-5.) CONH2 Cl NO2 (d) Aklomide CN Cl NO2 (c) Br Cl NO2 (b) Br NH2NH2 (a) C h a p t e r 2 2 1073 47. Explain the mechanism of the following synthetic transformation. (Hint: Two equivalents of butyllithium are used.) CH2CH2CH2CH3 CH2OH OCH3 1. CH3CH2CH2CH2Li 2. H2C 3. H�, H2O F O OCH3 P 48. In nucleophilic aromatic substitution reactions that proceed by the addition – elimination mechanism, fl uorine is the most easily replaced halogen in spite of the fact that F2 is by far the worst leaving group among halide ions. For example, 1-fl uoro-2,4-dinitrobenzene reacts much more rapidly with amines than does the corresponding chloro compound. Suggest an explanation. (Hint: Consider the effect of the identity of the halogen on the rate-determining step.) 49. Based on the mechanism presented for the Pd-catalyzed reaction of a halobenzene with hydroxide ion, write out a reasonable mechanism for the Pd-catalyzed reaction of 1-bromo-3-methoxybenzene with 2-methyl-1-propanamine shown in Section 22-4. 50. Give the likely products of each of the following reactions. Each one is carried out in the presence of a Pd catalyst, a phosphine, and heat. (a) ClF3C N H � (b) Br CH3CH2CH2SH� (c) ICH3O Na� �CN� (d) Br � � O š 51. A very effi cient short synthesis of resveratrol (Chemical Highlight 22-1) was reported in 2006. Fill in reasonable reagents for steps (a) through (d). Refer to the referenced text sections as needed. O O HO HO CHO HO HO CH CH2P B B CH3CO CH3CO CH CH2P (a) Section 17-12 (b) Sections 19-9, 20-3 O O B B CH3CO O B OCCH3 resveratrol CH3CO CH CHP (c) Section 13-9 (d) Section 20-4 52. The reaction sequence shown below illustrates the synthesis of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), a powerful herbicide. A 1 : 1 mixture of the butyl esters of 2,4,5-T and its dichlo- rinated analog 2,4-D was used between 1965 and 1970 as a defoliant during the Vietnam War P r o b l e m s 1074 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s under the code name Agent Orange. Propose mechanisms for the reactions in the synthesis of this substance, whose effects on the health of those exposed to it remain topics of considerable controversy. Cl Cl 2,4,5-Trichlorophenol (2,4,5-TCP) Cl Cl OH Cl Cl Cl 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) 85% OCH2COOH Cl Cl Cl 1. NaOH, 150°C 2. H�, H2O �NaCl ClCH2COOH, NaOH, H2O, � �NaCl 53. Give the expected major product(s) of each of the following reactions and reaction sequences. (a) 1. KMnO4, �OH, � 2. H�, H2O (b) 1. MnO2, acetone 2. KOH, H2O, � B CH2OH CH2CCH3 O (c) 1. (CH3)2CHCl, AlCl3 2. HNO3, H2SO4 3. KMnO4, NaOH, � 4. H�, H2O CH3 54. Rank the following compounds in order of descending acidity. (a) CH3OH (b) CH3COOH (c) OH SO3H (d) OH OCH3 (e) OH CF3 (f) OH 55. Design a synthesis of each of the following phenols, starting with either benzene or any mono- substituted benzene derivative. (a) OH CH3 (b) OH BrBr (c) The three benzenediols (d) OH NO2 NO2Cl 56. Starting with benzene, propose syntheses of each of the following phenol derivatives. (a) OCH2COOH Cl Cl The herbicide 2,4-D (b) NHCOCH3 OCH2CH3 (The active ingredient in Midol) Phenacetin (c) OCCH3 COOH Br Br B O Dibromoaspirin (An experimental drug for the treatment of sickle-cell anemia) C h a p t e r 2 2 1075 58. Give the expected product(s) of each of the following reaction sequences. (a) 1. 2 CH2PCHCH2Br, NaOH 2. � OH OH (b) 1. � 2. O3, then Zn, H� 3. NaOH, H2O, � CH3 O (c) Ag2O OH OH Cl Cl Cl Cl (d) Ag2O CH3 OH OH H3C (e) O O CH3CH2SH (two possibilities) (f) O O � 57. Name each of the following compounds. (a) OH Cl Br (b) OH CH2OH (c) OHHO SO3H (d) O OH (e) O O SCH3 59. As a children’s medicine, acetaminophen (Tylenol) has a major marketing advantage over aspirin: Liquid Tylenol preparations (essentially, acetaminophen dissolved in fl avored water) are stable, whereas comparable aspirin solutions are not. Explain. 60. Chili peppers contain signifi cant quantities of vitamins A, C, and E, as well as folic acid and potassium. They also contain small quantities of capsaicin, the fi ery essence responsible for the “hot” in hot peppers. The pure substance is in fact quite dangerous: Chemists working with capsaicin must work in special fi ltered-air enclosures and wear full body protection. One milli- gram placed on the skin will cause a severe burn. Although capsaicin has no odor or fl avor of its own, its pungency — in the form of detectable stimulation of the nerves in the mucous membranes of the mouth — can be detected even when the substance is diluted to one part in 17 million of water. The hottest peppers exhibit about 1y20th of this level of pungency. The structure of capsaicin is shown on p. 1028. Some of the data that were used in its elu- cidation are presented below. Interpret as much of this information as you can. MS: mYz 5 122, 137 (base peak), 195 (tricky!), 305. IR: n~ 5 972, 1660, 3016, 3445, 3541 cm21. 1H NMR: d 5 0.93 (6H, d, J 5 8 Hz), 1.35 (2H, quin, J 5 7 Hz), 1.62 (2H, quin, J 5 7 Hz), 1.97 (2H, q, J 5 7 Hz), 2.18 (2H, t, J 5 7 Hz), 2.20 (1H, m), 3.85 (3H, s), 4.33 (2H, d, J 5 6 Hz), 5.33 (2H, m), 5.82 (2H, broad s), 6.73 (1H, d, J 5 9 Hz), 6.78 (1H, s), 6.85 (1H, d, J 5 9 Hz) ppm. 61. Biochemical oxidation of aromatic rings is catalyzed by a group of liver enzymes called aryl hydroxylases. Part of this chemical process is the conversion of toxic aromatic P r o b l e m s 1076 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s hydrocarbons such as benzene into water-soluble phenols, which can be easily excreted. However, the primary purpose of the enzyme is to enable the synthesis of biologically useful compounds, such as the amino acid tyrosine. CH2CHCOOH A NH2 CH2CHCOOH A NH2 O2, hydroxylase OH TyrosinePhenylalanine (a) Extrapolating from your knowledge of benzene chemistry, which of the following three possibilities seems most reasonable: The oxygen is introduced by electrophilic attack on the ring; the oxygen is introduced by free-radical attack on the ring; or the oxygen is introduced by nucleophilic attack on the ring? (b) It is widely suspected that oxacyclopropanes play a role in arene hydroxylation. Part of the evidence is the following observation: When the site to be hydroxylated is initially labeled with deuterium, a substantial proportion of the product still contains deuterium atoms, which have apparently migrated to the position ortho to the site of hydroxylation. R O2, hydroxylase D R OH D R O through H D as an intermediate Suggest a plausible mechanism for the formation of the oxacyclopropane intermediate and its conversion into the observed product. (Hint: Hydroxylase converts O2 into hydrogen peroxide, HO – OH.) Assume the availability of catalytic amounts of acids and bases, as necessary. Note: In victims of the genetically transmitted disorder called phenylketonuria (PKU), the hydroxylase enzyme system described here does not function properly. Instead, phenylalanine in the brain is converted into 2-phenyl-2-oxopropanoic (phenylpyruvic) acid, the reverse of the process shown in Problem 57 of Chapter 21. The buildup of this compound in the brain can lead to severe retardation; thus people with PKU (which can be diagnosed at birth) must be restricted to diets low in phenylalanine. 62. A common application of the Cope rearrangement is in ring-enlargement sequences. Fill in the reagents and products missing from the following scheme, which illustrates the construction of a 10-membered ring. (a) 1. LiAlH4, (CH3CH2)2O 2. H�, H2O(b) Excess PCC, CH2Cl2 200�C(e) O O O O O O (c) (f)(d) C h a p t e r 2 2 1077 67. Write the most reasonable structure of the product of each of the following reaction sequences. 63. As mentioned in Section 22-10, the Sandmeyer reactions, in which copper(I) ion catalyzes substitution of the nitrogen in arenediazonium salts by Cl, Br, or CN, take place by complex mechanisms involving radicals. Explain why these substitutions do not follow either the SN1 or SN2 pathway. 64. Formulate a detailed mechanism for the diazotization of benzenamine (aniline) in the presence of HCl and NaNO2. Then suggest a plausible mechanism for its subsequent conversion into iodobenzene by treatment with aqueous iodide ion (e.g., from K1I2). Bear in mind your answer to Problem 63. 65. Show how you would convert 3-methylbenzenamine into each of the following compounds: (a) methylbenzene; (b) 1-bromo-3-methylbenzene; (c) 3-methylphenol; (d) 3-methylbenzonitrile; (e) N-ethyl-3-methylbenzenamine. 66. Devise a synthesis of each of the following substituted benzene derivatives, starting from benzene. (a) Cl Br (b) CN COOH (c) OH NO2 O2N Cl (d) OH CN (e) COOH I (f) Cl Br Cl Br (g) COOH Br Br Br (a) NH2 SO3H 1. NaNO2, HCl, 5�C 2. HO OH Golden Yellow (b) NH2 SO3H 1. NaNO2, HCl, 5�C 2. NH Metanil Yellow For the following reaction, assume that electrophilic substitution occurs preferentially on the most activated ring. (c) OH SO3H Orange I 1. NaNO2, HCl, 5°C 2. NH2 P r o b l e m s 1078 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 68. Show the reagents that would be necessary for the synthesis by diazo coupling of each of the following three compounds. For structures, see Section 22-11. (a) Methyl Orange (b) Congo Red (c) Prontosil, H2N NH2 SO2NH2 N N J , which is converted microbially into sulfanilamide, H2N SO2NH2 (The accidental discovery of the antibacterial properties of prontosil in the 1930s led indirectly to the development of sulfa drugs as antibiotics in the 1940s.) 69. (a) Give the key reaction that illustrates the inhibition of fat oxidation by the pre- servative BHT. (b) The extent to which fat is oxidized in the body can be determined by measur- ing the amount of pentane exhaled in the breath. Increasing the amount of vitamin E in the diet decreases the amount of pentane exhaled. Examine the processes described in Section 22-9 and identify one that could produce pentane. You will have to do some extrapolating from the specifi c reactions shown in the section. 70. The urushiols are the irritants in poison ivy and poison oak that give you rashes and make you itch upon exposure. Use the following information to determine the structures of urushiols I (C21H36O2) and II (C21H34O2), the two major members of this family of unpleasant compounds. Urushiol II urushiol I H2, Pd–C, CH3CH2OH 1. O3, CH2Cl2 2. Zn, H2OUrushiol II C23H38O2 Excess CH3I, NaOH Dimethylurushiol II Aldehyde A CH3CH2CH2CH2CH2CH2CHO C16H24O3� OCH3 Synthesis of Aldehyde A 1. SO3, H2SO4 2. HNO3, H2SO4 1. H2, Pd, CH3CH2OH 2. NaNO2, H�,H2O 3. H2O, �H�, H2O, �C7H7NSO6 C7H7NO3 B C 1. CO2, pressure, KHCO3, H2O 2. NaOH, CH3I 3. H�, H2O 1. LiAlH4, (CH3CH2)2O 2. H�, H2O 3. MnO2, acetoneC7H8O2 D C9H10O4 E 1. C6H5CH2O(CH2)6CH 2. Excess H2, Pd–C, CH3CH2OH 3. PCC, CH2Cl2 C9H10O3 F aldehyde A PP(C6H5)3 71. Is the site of reaction in the biosynthesis of norepinephrine from dopamine (see Chapter 5, Problem 65) consistent with the principles outlined in this chapter? Would it be easier or more diffi cult to duplicate this transformation nonenzymatically? Explain. C h a p t e r 2 2 1079 Team Problem 72. As a team, consider the following schemes that outline steps toward the total synthesis of taxo- done D, a potential anticancer agent. For the fi rst scheme, divide your team into two groups, one to discuss the best option for A to effect the initial reduction step, the second to assign a structure to B, using the partial spectral data provided. OHO OCCH3 H3C CH3CO O H % ≥ B O B OCCH3 B H3C CH3CO O H % ≥ B O B A H�, toluene, � �1H NMR: 5.99 (dd, 1 H), 6.50 (d, 1 H) ppm. IR: ˜� 1720 cm�1. MS: m/z 384 (M�). Reconvene to discuss both parts of the fi rst scheme. Then, as a group, analyze the remainder of the synthesis shown in the second scheme below. Use the spectroscopic data to help you determine the structures of C and taxodone, D. UV-Vis: �max ( ) 316 (20,000) nm. MS: m/z 316 (M�). DB C 1H NMR: 3.51 (dd, 1 H), 3.85 (d, 1 H) ppm. MS: m/z 400 (M�). � �OH, H2O COOH O Cl B 1H NMR: 6.55 (d, 1 H), 6.81 (s, 1 H) ppm, no other alkenyl or aromatic signals. IR: ˜ � � 3610, 3500, 1628 cm�1. Taxodone Propose a mechanism for the formation of D from C. (Hint: After ester hydrolysis, one of the phenoxide oxygens can donate its electron pair through the benzene ring to effect a reaction at the para position. The product contains a carbonyl group in its enol form. For the interpretation of some of the spectral data, see Section 17-3.) Preprofessional Problems 73. After chlorobenzene has been boiled in water for 2 h, which of the following organic compounds will be present in greatest concentration? P r o b l e m s (a) C6H5OH (b) OH Cl (c) OH Cl (d) C6H5Cl (e) HCl O 74. What are the products of the following reaction? C6H5OCH3 uy? HI, DHI, D (a) C6H5I 1 CH3OH (b) C6H5OH 1 CH3I (c) C6H5I 1 CH3I (d) OCH3 H2 I � 1080 C h a p t e r 2 2 C h e m i s t r y o f B e n z e n e S u b s t i t u e n t s 75. The transformation of 4-methylbenzenediazonium bromide to toluene is best carried out by using: (a) H1, H2O (b) H3PO2, H2O (c) H2O, 2OH (d) Zn, NaOH 76. What is the principal product after the slurry obtained on treating benzenamine (aniline) with potassium nitrite and HCl at 08C has been added to 4-ethylphenol? (a) N CH2CH3 NC6H5 OH P (b) NHC6H5 CH2CH3 OH (c) N CH2CH3 OH NC6H5P (d) C6H5 CH2CH3 OH 77. Examination of the 1H NMR spectra of the following three isomeric nitrophenols reveals that one of them displays a hydroxy (phenolic) proton at substantially lower fi eld than do the other two. Which one? (a) OH NO2 (b) OH NO2 (c) OH NO2 Chapter 22: CHEMISTRY OF BENZENE SUBSTITUENTS 22-1: Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization 22-2: Benzylic Oxidations and Reductions 22-3: Names and Properties of Phenols Chemical Highlight 22-1: Two Phenols in the News: Bisphenol A and Resveratrol 22-4: Preparation of Phenols: Nucleophilic Aromatic Substitutionnull 22-5: Alcohol Chemistry of Phenols Chemical Highlight 22-2: Aspirin: A Phenyl Alkanoate Drug 22-6: Electrophilic Substitution of Phenols 22-7: An Electrocyclic Reaction of the Benzene Ring: The Claisen Rearrangement 22-8: Oxidation of Phenols: Benzoquinones Chemical Highlight 22-3: Chemical Warfare in Nature: The Bombardier Beetle 22-9: Oxidation-Reduction Processes in Nature 22-10: Arenediazonium Salts 22-11: Electrophilic Substitution with Arenediazonium Salts: Diazo Coupling Chemical Highlight 22-4: William Perkin and the Origins of Industrial and Medicinal Chemistry Chapter Integration Problems New Reaction Important Conceptsnull Problems