Blanc chloromethylation

The Blanc chloromethylation is the chemical reaction of aromatic rings with formaldehyde and hydrogen chloride catalyzed by zinc chloride to form chloromethyl arenes. The reaction was discovered by Gustave Louis Blanc (1872-1927) in 1923.^[1][2][3] The reaction should be performed with care, as (like most chloromethylation reactions) it produces highly carcinogenic bis(chloromethyl) ether as a by-product.

Mechanism

The reaction is carried out under acidic conditions and with a ZnCl₂ catalyst. These conditions protonate the formaldehyde carbonyl making the carbon much more electrophilic. The aldehyde is then attacked by the aromatic pi-electrons, followed by rearomatization of the aromatic ring. The benzyl alcohol thus formed is quickly converted to the chloride under the reaction conditions.

Although the reaction is an efficient means of introducing a chloromethyl group, the production of small amounts of highly carcinogenic bis(chloromethyl) ether is a disadvantage.

See also

• Friedel-Crafts alkylation
• Quelet reaction
• Bouveault–Blanc reduction

References

1. Gustave Louis Blanc Bull. Soc. Chim. France 1923, 33, 313
2. Whitmore, F. C.; Ginsburg, Abram; Rueggeberg, Walter; Tharp, I.; Nottorf, H.; Cannon, M.; Carnahan, F.; Cryder, D.; FLeming, G.; Goldberg, G.; Haggard, H.; Herr, C.; Hoover, T.; Lovell, H.; Mraz, R.; Noll, C.; Oakwood, T.; Patterson, H.; Van Strien, R.; Walter, R.; Zook, H.; Wagner, R.; Weisgerber, C.; Wilkins, J. (May 1946). "Production of Benzyl Chloride by Chloromethylation of Benzene. Laboratory and Pilot Plant Studies". Industrial & Engineering Chemistry 38 (5): 478–485. doi:10.1021/ie50437a013.
3. Belen'kii, Leonid I; Vol'kenshtein, Yu B; Karmanova, I B (30 September 1977). "New Data on the Chloromethylation of Aromatic and Heteroaromatic Compounds". Russian Chemical Reviews 46 (9): 891–903. doi:10.1070/RC1977v046n09ABEH002180.