The Blanc reaction allows the chloromethylation of aromatic compounds.

It uses as reagents methane with gaseous hydrogen chloride in the presence of a Lewis acid. The result is the introduction of a hydroxymethyl group on the aromatic ring (benzene) whose hydroxyl is replaced by chlorine in the presence of hydrogen chloride.

Mechanism:

Stage 1. Protonation of methanal

Stage 2. Nucleophilic attack of benzene to the electrophile.

Stage 3. Recovery of aromaticity.

Stage 4. Substitution of hydroxyl by chlorine by means of S_N1.

Chloromethylation can be used with aromatic compounds in general: benzene, naphthalene, anthracene. The benzyl chlorides obtained can be transformed into numerous derivatives: aldehydes, ketones, nitriles, organometallic reagents, etc.