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The Blanc reaction, also called Blanc chloromethylation, is the chemical reaction of aromatic rings with formaldehyde and hydrogen chloride catalyzed by zinc chloride or other Lewis acids to form chloromethyl arenes.
The reaction is carried out under acidic conditions, with ZnCl2 used as catalyst. The acidic medium protonates the formaldehyde carbonyl making the carbon much more electrophilic. The aldehyde is then attacked by the aromatic π-electrons, followed by rearomatization of the aromatic ring. The benzyl alcohol thus formed is quickly converted to the chloride under the reaction conditions.
Aryl thiols can also be chlorinated this way.
In a reaction flask, the aromatic compound and formaldehyde are added, the latter either as aqueous solution or in the form of paraformaldehyde, followed by zinc chloride. The flask is heated on a hot plate or some other heating bath. Hydrogen chloride gas is passed through the hot solution until no more gas is absorbed. The crude aryl chloride is purified.
Like other chloromethylation reactions, the Blanc reaction produces the highly carcinogenic bis(chloromethyl) ether as a by-product.