This reaction, which is comparable to a Friedel-Crafts Alkylation, is useful for the preparation of chloromethylated arenes (for example, the Merrifield resin based on polystyrene) from the parent arene with formaldehyde, HCl, and ZnCl2.
Mechanism of the Blanc Reaction
The Lewis acid ZnCl2 effects formation of an oxonium ion which is reactive in electrophilic aromatic substitution. The intermediate zinc alkoxide reacts with the arene to form the chloromethylated product and zinc oxides:
When the concentration (or, effective concentration in the case of polymer residues) is high, the formation of side products due to a second addition are observed: