Blanc Chloromethylation

The Blanc chloromethylation, also known as the Blanc reaction, is a chemical process that involves the addition of a chloromethyl group onto aromatic rings using formaldehyde and hydrogen chloride. This reaction is catalyzed by Lewis acids, with zinc chloride being a commonly used catalyst.

The reaction takes place in an acidic environment with the presence of a ZnCl2 catalyst. These conditions result in the protonation of the carbonyl group of formaldehyde, significantly increasing the electrophilicity of the carbon atom. The aromatic ring's pi-electrons then attack the activated aldehyde, leading to the formation of a new carbon-carbon bond. Subsequently, the aromatic ring undergoes rearomatization, restoring its original aromaticity. The resulting intermediate, benzyl alcohol, readily reacts under the reaction conditions to form the corresponding chloride.

In summary, the Blanc chloromethylation reaction proceeds through the protonation of formaldehyde, the attack of the aromatic ring on the activated aldehyde, and the subsequent formation of a chloromethylated aromatic compound. This transformation is facilitated by the presence of an acidic medium and the Lewis acid catalyst zinc chloride.

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[1]Franke A, Mattern G, Traber W. Synthetische Juvenilhormone 3. Mitteilung. p‐Substituierte 4‐Phenyl‐buten (1)‐carbonsäurederivate[J]. Helvetica Chimica Acta, 1975, 58(1): 283-292.

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