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1 CHAPTER 22 Chemistry of Benzene Substituents: Alkylbenzenes, Phenols and Benzenamines Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization 22-1 The methyl C-H bonds in methylbenzene are relatively weak with respect to homolytic and heterolytic cleavage. The phenylmethyl (benzyl) group may be viewed as a benzene ring whose π system overlaps with an extra p orbital on the attached alkyl carbon: Reactivity at the Phenylmethyl (Benzyl) Carbon: Benzylic Resonance Stabilization 22-1 Benzylic radicals are reactive intermediates in the halogenation of alkylbenzenes. Benzene will not react with Cl2 or Br2 unless a Lewis acid is added: Heat or light allows attack of Cl2 or Br2 on methylbenzene even in the absence of a catalyst, however, attack is at the methyl group, not the aromatic ring. Excess halogen leads to multiple substitution. The mechanism of benzylic halogenation proceeds through radical intermediates: The benzylic C-H bond is relatively weak (DHo=87 kcal mol-1) due to resonance stabilization of the intermediate radical formed. Subsequent halogen attack is always at the benzylic position because attack at an aromatic carbon would destroy the aromatic character of the ring
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2 Benzylic cations delocalize the positive charge. Benzylic resonance can strongly affect the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, a primary benzylic tosylate rapidly reacts with ethanol via an SN1 reaction: Delocalization of positive charge into the aromatic ring facilitates the dissociation of the starting sulfonate: Several benzylic cations are stable enough to have been isolated. The X-ray structure of 2-phenyl-2-propyl cation (as its SbF6 - salt) was obtained in 1997. A para methoxy substituent on the benzene ring allows for extra stabilization of the benzylic positive charge. In its absence, the SN2 reaction may dominate due to the lack of steric interference and the stabilization of the SN2 transition state by overlap with the benzene π system. Resonance in benzylic anions makes benzylic hydrogens relatively acidic. The anion, radical and cation adjacent to a benzene ring are all stabilized by conjugation: The acidity of methylbenzene (pKa~41) is considerably greater than that of ethane (pKa~50) and comparable to that of propene (pKa~40) which can be deprotonated to form the resonancestabilized 2-propenyl anion. Consequently, methylbenzene can be deprotonated by butyllithium to generate phenylmethyllithium:
22-2 Benzvlic Oxidations and Reductions Bdasinaieatagadbenzenesleadstn u w8goa nzylic ethers are cleaved by hydrogenolysis H OH HOR 网 22-3 Names and Properties of Phenols ---5-d 中 3
3 22-2 Benzylic Oxidations and Reductions Oxidation of alkyl-substituted benzenes leads to aromatic ketones and acids. Hot KMnO4 and Na2Cr2O7 may oxidize alkylbenzenes all the way to benzoic acids. These reactions require at least one benzylic CH bond to be present in the starting materials (tertiary alkylbenzenes are inert). The oxidation reaction proceeds through the alcohol, the ketone and then the acid. It can be stopped at the ketone stage under milder conditions. Benzylic alcohols, in the presence of other non-benzylic hydroxy groups, can be oxidized to the corresponding carbonyl compounds under mild conditions. Benzylic ethers are cleaved by hydrogenolysis. Exposure of benzylic alcohols or ethers to hydrogen in the presence of a metal catalyst leads to cleavage of a σ-bond by catalytically activated hydrogen. Since hydrogenolysis is not possible for ordinary alcohols and ethers, the phenylmethyl substituent is a valuable protecting group for hydroxy functions. Protection by a tertiary butyl group would require acid to cleave, which might cause dehydration. 22-3 Names and Properties of Phenols In hydroxy-substituted arenes (phenols), the π system of the benzene ring overlaps an occupied p orbital on the oxygen atom. This results in delocalization similar to that found in benzylic anions. Enols are usually unstable and revert to their ketone forms. Phenols, however prefer the enol form which preserves the aromatic nature of the aromatic ring
22-3 Names and Properties of Phenols of phenol derivative)are gredphenehndnhotogrpy.yen9an H- 8 cal activity are: ally ac 脱o0 epnateoahenotsdetoresonmncestabtiatoncd 5aa 4
4 22-3 Names and Properties of Phenols Phenols are hydroxyarenes. Phenol was formerly known as “carbolic acid.” Aqueous solutions of phenol (or its derivatives) are used as disinfectants. Its main use is in the preparation of polymers (phenolic resins). Pure phenol is toxic and causes severe skin burns. Substituted phenols are named as phenols, benzenediols or benzenetriols. Some common names are accepted by IUPAC. Substituted phenols find uses in photography, dyeing and tanning. Bisphenol A is an important monomer in the synthesis of epoxyresins and polycarbonates. Phenols containing a carboxylic acid functionality (higher ranking) are called hydroxybenzoic acids. Phenyl ethers are named as alkoxybenzenes. As a substituent, C6H5O is called phenoxy. Examples of phenols possessing physiological activity are: Phenols are unusually acidic. The pKa values of phenols range from 8 to 10. They are less acidic than carboxylic acids (pKa=4-5) and stronger than alkanols (pKa=16-18). The acidic nature of phenols is due to resonance stabilization of the phenoxide ion: Substituents can affect the acidity of phenols: Multiple nitrations can increase the acidity to that of carboxylic or even mineral acids. Electron donating substituents have the opposite effect:
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5 Preparation of Phenols: Nucleophilic Aromatic Substitution 22-4 Nucleophilic aromatic substitution may follow an addition-elimination pathway. Displacement of a group (other than hydrogen) from an aromatic ring is called ipso substitution. The transformation is called nucleophilic aromatic substitution. Success of this reaction is dependent upon the presence of one or stronger electron-withdrawing groups located on the ring ortho or para to the leaving group. Electron-withdrawing groups stabilize the intermediate anion by resonance. Nucleophilic aromatic substitution reactions proceed by a twostep addition-elimination sequence. In the meta compound, 1-chloro-3,5-dinitrobenzene, ipso substitution does not occur: The reactivity of haloarenes in nucleophilic substitutions increases with the nucleophilicity of the reagent and the number of electron-withdrawing groups on the ring. Haloarenes may react through benzyne intermediates. Haloarenes without electron-withdrawing substituents can undergo nucleophilic substitution at highly elevated temperatures and pressures
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6 When radioactive chlorobenzene is reacted with KNH2, as in the previous reaction, a curious result is obtained: The mechanism of this reaction involves the base-induced elimination of HX from the benzene ring to give alkynes. Both reactions of Step 1 are difficult: •The acidity of the phenyl C-H bond is very low. •The resulting negative charge upon deprotonation cannot be resonance-stabilized since the sp2 orbital is perpendicular to the aromatic π frame. •The resulting benzyne intermediate is very highly strained. Benzyne is a strained cycloalkyne. Benzyne can be observed spectroscopically under special conditions but is too unstable to be isolated. It can be observed in the IR and UV spectra of benzocyclobutenedione, which has undergone photolysis in frozen argon. The triple bond in benzyne exhibits an IR-stretching frequency of 1846 cm-1, intermediate between that of a normal double bond (~1652 cm-1) and a normal triple bond (~2207 cm-1). The 13C NMR spectra for the benzyne carbons occurs at δ = 182.7 ppm, also a typical of pure triple bonds. The bond is weakened by poor p-orbital overlap in the plane of the ring. Phenols are produced from arenediazonium salts. The usual laboratory procedure for synthesizing phenols is through arenediazonium salts, ArN2 +X- . Primary benzenamines (anilines) are attacked by cold nitrous to give relatively stable arenediazonium salts in a reaction called diazotization. When arenediazonium ions are heated, nitrogen is evolved and reactive aryl cations are produced, which are then trapped by water to form phenols
22-5 Alcohol Chemistry of Phenols otoommphen =+ kexybenzenes are prepared by Williamson ether erification leads to phenyl alkanoates. haloalkanes 22-6 Electrophilic Substitution of Phenols 7
7 22-5 Alcohol Chemistry of Phenols The oxygen in phenols is only weakly basic. Phenols can be protonated by strong acids to give phenyloxonium ions. The pKa’s for phenyloxonium ions are lower than those of alkyloxonium ions, however, because the oxygen lone electron pairs are delocalized into the benzene ring. Phenyloxonium derivatives do not dissociate to form phenyl cations: The energy of such ions is too high. Protonation of alkoxybenzenes, however, allows the bond between the oxygen and the alkyl group to be readily cleaved in the presence of nucleophiles such as Br- or I- . Alkoxybenzenes are prepared by Williamson ether synthesis. Many alkoxybenzenes can be prepared using the Williamson ether synthesis since the phenoxide ions obtained by deprotonation of phenols are good nucleophiles. Phenoxides can displace the leaving groups from haloalkanes and alkyl sulfonates. Esterification leads to phenyl alkanoates. Esterification of carboxylic acids with phenols requires an activated carboxylic acid derivative such as an alkanoyl halide or a carboxylic anhydride. 22-6 Electrophilic Substitution of Phenols The OH group in a phenol strongly activates the ortho and para position towards electrophilic substitution. Dilute nitric acid causes nitration: Friedel-Crafts alkanoylation of a phenol is better carried on an ether derivative of the phenol to avoid ester formation:
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8 Halogenation of phenols occurs readily and a catalyst is not required. Multiple halogenations are frequently observed but can be controlled by using a lower temperature and a less polar solvent. Para electrophilic attack frequently dominates because of steric effects. The ratio of para to ortho products is highly dependent upon reagents and reaction conditions. Under basic conditions, phenols can undergo electrophilic substitution through intermediate phenoxide ions, even with very mild electrophiles. The initial aldol products are unstable and dehydrate upon heating, giving reactive intermediates called quinomethanes. Quinomethanes are α,β-unsaturated carbonyl compounds, which can undergo Michael additions with excess phenoxide ion. Subsequent repeated hydroxymethylation and Michael addition eventually leads to the formation of a complex phenolformaldehyde copolymer called a phenolic resin. The precursor to asprin, o-hydroxybenzoic acid, can be prepared via the Kolbe reaction: An Electrocyclic Reaction of the Benzene Ring: The Claisen Rearrangement 22-7 The Claisen rearrangement is a concerted reaction with a transition state that accommodates the movement of six electrons
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9 With the non-aromatic 1-ethenyloxy-2-propene system, the rearrangement stops at the carbonyl stage because there is no driving force for enolization. This is called the aliphatic Claisen rearrangement. The carbon analog of the Claisen rearrangement takes place in compounds containing 1,5-diene units. It is called the Cope rearrangement: All of the preceding rearrangements are related to electrocyclic reactions that interconvert cis-1,3,5-hexatriene with 1,3- cyclohexadiene. The only difference is the absence of a double bond connecting the terminal π bonds. Oxidation of Phenols: Cyclohexadienediones (Benzoquinones) 22-8 Cyclohexadienediones (benzoquinones) and benzenediols (hydroquinones and catechols) are redox couples. The phenols 1,2- and 1,4-benzenediol are oxidized to their corresponding diketones by oxidizing agents such as sodium dichromate or silver oxide. When the resulting diones are reactive, as for o-benzoquinone, yields may be variable. The mechanism or the redox process that interconverts hydroquinone and p-benzoquinone passes through a radical intermediate: Redox processes similar to this occur widely in nature. The enone units in 2,5-cyclohexadiene-1,4-diones (p-benzoquinones) undergo conjugate and DielsAlder additions. p-Benzoquinones function as reactive α,β-unsaturated ketones in conjugate addition reactions:
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10 The double bonds in α,β-unsaturated ketones also undergo cycloadditions to dienes: 22-9 Oxidation-Reduction Processes in Nature Ubiquinones mediate the biological reduction of oxygen to water. Nature utilizes the benzoquinone-hydroquinone redox couple in the reversible oxidation reactions of the cascade of steps by which molecular oxygen is used in biochemical degradations. The ubiquinones, collectively called coenzyme Q, CoQ or Q, are an important series of compounds used for this purpose. An enzyme system that utilized NADH, a biological reductant, converts Q into its reduced form, QH2. QH2 then participates in a series of reactions involving the cytochromes (electron-transporting iron containing proteins) and ending with the reduction of O2 to water: Phenol derivatives protect cell membranes from oxidative damage. Two intermediates in the conversion of molecular oxygen into water are the highly reactive radicals superoxide, O2 - ·, and the hydroxyl radical, ·OH, which arises from the cleavage of H2O2. An example of the damage caused by reactive radicals is illustrated by the reaction with a phosphoglyceride containing an unsaturated fatty acid: Cleavage of the relatively weak O-O bond gives rise to an alkoxy radical which may decompose (β-scission) to an unsaturated aldehyde. Related, but more complex mechanisms, yield unsaturated hydroxyaldehydes, such as trans-4-hydroxy-2-nonenal and propanedial