31.1.1—Preparation of halogenoarenes

Benzene or methylbenzene reacts with Cl₂ or Br₂ in the presence of AlCl₃ or AlBr₃. The catalyst polarises the halogen and enables electrophilic substitution on the ring.

The arene loses H⁺ and retains aromaticity after substitution. Methylbenzene gives mainly the 2- and 4-substituted products because the methyl group directs electrophiles.

Benzene + Cl₂/AlCl₃ → chlorobenzene + HCl. Methylbenzene under the same type of conditions gives 2-chloromethylbenzene and 4-chloromethylbenzene.

Without the Lewis acid, simply mixing benzene and chlorine is not the syllabus preparation; do not confuse ring halogenation with UV side-chain substitution.