31.1 Halogen compounds

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.

The C–Cl bond in chlorobenzene is less susceptible to nucleophilic substitution than the C–Cl bond in chloroethane. In chlorobenzene, the chlorine lone pair overlaps with the aromatic π system, giving the bond partial double-bond character.

The aryl carbon is sp² and the bond is shorter and stronger; attack is also hindered by the ring. Chloroethane has a more accessible sp³ carbon with a normal polar C–Cl bond.

Aqueous hydroxide readily substitutes chloroethane on heating, whereas chlorobenzene needs much harsher conditions and is not treated as an ordinary halogenoalkane.

Both molecules contain C–Cl, but functional context changes reactivity; do not transfer the halogenoalkane mechanism automatically to an arene.