Substitution Reactions of Benzene

Because of its greater stability than alkenes benzene tends to undergo substitution reactions rather than addition reactions (as do the alkenes).

Nitration. Benzene reacts with a mixture of concentrated nitric and sulfuric acids to produce nitrobenzene as shown in the reaction below. The process is termed nitration. Sulfuric acid is the catalyst and the mixture must be warmed to about 500^оC to initiate the reaction.

Halogenation. Both bromine and chlorine react with benzene if iron is present. The actual catalyst is FeCl₃ or FeBr₃ (metallic iron also works as a catalyst because it reacts with halogens to produce the above salts). The substitution by bromine, called bromination is given as an example below. Notice that only one atom of bromine reacts with the benzene, the other combines with the displace hydrogen to form HBr which escapes as a gas.

Alkylation (Friedel-Crafts reaction) Substitution of an alkyl group onto a benzene ring is called alkylation. The reaction is very similar to halogenations except that aluminum chloride is used to catalyze the reaction, as shown in the reaction below. In general alkyl halide RX is used in the substitution to produce an alkyl-benzene.

29.Theme: Aromatic hydrocarbons. Benzene and derivatives of benzene.