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Concept 06 / 07Hydrocarbons

Directive Influence of Substituents in Benzene and Carcinogenicity of Arenes

How existing substituents control rate and position of electrophilic substitution: activating ortho/para directors, deactivating ortho/para directing halogens, and deactivating meta directors, explained with resonance structures of the arenium ion; worked orientation problems; and carcinogenic polynuclear aromatic hydrocarbons.

Figure 1
When a substituted benzene undergoes electrophilic substitution, the existing group decides both how fast the reaction goes (activation or deactivation relative to benzene) and where the new group enters (see figure). Ortho/para directing, activating groups. Groups with a lone pair on the atom attached to the ring donate electrons by resonance (+R+R): −NH2\mathrm{-NH_{2}}, −NHR\mathrm{-NHR}, −OH\mathrm{-OH}, −OR\mathrm{-OR}, −NHCOCH3\mathrm{-NHCOCH_{3}}. Alkyl groups activate by the +I+I effect and hyperconjugation. Resonance structures of the substituted ring show increased electron density at ortho and para positions. More decisively, when E+\mathrm{E^{+}} attacks ortho or para, one resonance form of the arenium ion places the positive charge on the carbon bearing the substituent, where a lone pair can stabilise it (an extra resonance structure with complete octets). Attack at meta gives no such form. So ortho and para products dominate, and the reaction is faster than with benzene: phenol is brominated by bromine water without a catalyst to 2,4,6-tribromophenol. Ortho/para directing, deactivating groups: halogens. Halogens withdraw electrons strongly by induction (−I-I), which deactivates the ring (chlorobenzene reacts more slowly than benzene), but their lone pairs donate by resonance (+R+R) to stabilise the ortho and para arenium ions. So they direct ortho/para while deactivating. Meta directing, deactivating groups. Groups with a positive or partially positive atom attached to the ring withdraw electrons by −I-I and −R-R: −NO2\mathrm{-NO_{2}}, −CN\mathrm{-CN}, −CHO\mathrm{-CHO}, −COR\mathrm{-COR}, −COOH\mathrm{-COOH}, −COOR\mathrm{-COOR}, −SO3H\mathrm{-SO_{3}H}, −NR3+\mathrm{-NR_{3}^{+}}, −CCl3\mathrm{-CCl_{3}}. Ortho or para attack would put the arenium positive charge right next to the electron-poor substituent (very unfavourable); meta attack avoids this. So the meta product dominates, formed more slowly than from benzene: nitration of nitrobenzene needs fuming acids and heat, giving m-dinitrobenzene. Orientation rules in disubstitution. With two groups, the more strongly activating group usually controls; substitution between two meta-related groups is sterically hindered. In strongly acidic media, −NH2\mathrm{-NH_{2}} is protonated to −NH3+\mathrm{-NH_{3}^{+}}, a meta director: nitration of aniline gives a large amount (about 47%47\%) of m-nitroaniline, besides oxidation products. Acetylation to acetanilide moderates the group and gives mainly p-nitroacetanilide. Carcinogenicity and toxicity. Benzene and polynuclear aromatic hydrocarbons with more than two fused rings are toxic and carcinogenic. They form from incomplete combustion of tobacco, coal and petroleum. Examples: 1,2-benzanthracene, 3-methylcholanthrene, 1,2,5,6-dibenzanthracene, 3,4-benzpyrene (benzo[a]pyrene). They are metabolised to reactive species that damage DNA. Key Takeaways: - +R+R and alkyl groups: activating, ortho/para. - Halogens: deactivating (−I-I) but ortho/para (+R+R). - −NO2\mathrm{-NO_{2}}, −CN\mathrm{-CN}, −CHO\mathrm{-CHO}, −COOH\mathrm{-COOH}, −SO3H\mathrm{-SO_{3}H}, −NR3+\mathrm{-NR_{3}^{+}}: deactivating, meta. - Reactivity: phenol > toluene > benzene > chlorobenzene > nitrobenzene. - Pitfall: aniline in strong acid behaves as the anilinium ion, a meta director. Brain-Teaser 1: Arrange in decreasing reactivity towards nitration: benzene, nitrobenzene, toluene, chlorobenzene, phenol. Solution: Phenol > toluene > benzene > chlorobenzene > nitrobenzene. OH is strongly +R+R; CH3\mathrm{CH_{3}} is weakly activating; Cl deactivates by −I-I; NO2\mathrm{NO_{2}} deactivates strongly by −I-I and −R-R. Brain-Teaser 2: Suggest a sequence to make m-bromonitrobenzene and p-bromonitrobenzene from benzene. Solution: The order of steps controls orientation. For meta: nitrate first (nitro is meta-directing), then brominate with Br2\mathrm{Br_{2}}/FeBr3\mathrm{FeBr_{3}}. For para: brominate first (Br is ortho/para-directing), then nitrate, separating the para isomer from the ortho. Brain-Teaser 3: Why does bromine water give 2,4,6-tribromophenol with phenol but no reaction with benzene? Solution: The OH group strongly activates the ring by +R+R; in water, phenol partly ionises to phenoxide, which is even more activating. The ring is so electron-rich that polarised Br2\mathrm{Br_{2}} substitutes at all free ortho and para positions without a Lewis acid. Benzene needs FeBr3\mathrm{FeBr_{3}} to generate a sufficiently strong electrophile.

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