Hell–Volhard–Zelinsky reaction is a method to convert carboxylic acids bearing α hydrogen to α-brominated carboxylic acids in the presence of a mixture of phosphorus tribromide and bromine, followed by hydrolysis.

Phosphorus tribromide acts as a catalyst—it is either added directly or generated in situ by the reaction of red phosphorus with bromine.

The reaction begins with the carboxylic acid and the catalyst interacting to generate the acid bromide and HBr.

HBr then catalyzes the enolization of acid bromide to its enol form, both of which exist in equilibrium.

The enol form acts as a nucleophile that undergoes an α substitution reaction with bromine to form an α-bromo acid bromide.

This active intermediate, upon treatment with water, undergoes hydrolysis to produce the desired α-brominated acid.

If instead of water, alcohols or amines are used, the intermediate is converted to its respective α-brominated acid derivative.