7.6.1 Halogenation

Halogens react with benzene in the presence of a strong Lewis acid such as AlCl₃ or FeBr₃ to give halobenzenes. For example, bromobenzene can be prepared with good yield as shown in Scheme 4.

Scheme 4

Bromine itself is not sufficiently electrophilic to react with benzene so that a strong Lewis acid such as FeBr₃ used as a catalyst for the formation of Br⁺ which attacks benzene to form the sigma complex. Bromide ion acts as a weak base to remove a proton from the sigma complex, giving the substituted benzene and HBr (Scheme 5).

Scheme 5

Chlorination of benzene works much like bromination. Aluminum chloride (A1Cl3) is often used as the Lewis acid catalyst for chlorination of benzene. Iodination of benzene requires an acidic oxidizing agent, such as nitric acid. The iodine cation, an electrophile, results from oxidation of iodine by nitric acid (Scheme 6).

Scheme 6