The Hell-Volhard-Zelinsky halogenation reaction halogenates carboxylic acids at the α carbon.

Typically, a carboxylic acid is first reacted with PBr₃ and Br₂. The PBr₃ replaces the OH group of the carboxylic acid with a bromine, resulting in a carboxylic acid bromide. The acid bromide can then tautomerize to an enol, which will readily react with the Br₂ to brominate the molecule a second time at the α position. The first step thus results in a carboxylic acid bromide which is also brominated at the α position.

In a second step, the carboxylic acid may be recovered by reacting the carboxylic acid bromide with water or a weak aqueous acid. This results in the hydrolysis of the acid bromide, i.e. the displacement of the bromine directly bound to the carbonyl group by an OH group. (The hydrolysis is an example of a nucleophilic acyl substitution.)

The final product is thus the original carboxylic acid brominated at the α position.

References

Organic reactions