Alpha Bromination of Carboxylic Acids

Although the alpha bromination of some carbonyl compounds, such as aldehydes and ketones, can be accomplished with Br₂ under acidic conditions, the reaction will generally not occur with acids, esters, and amides. This is because only aldehydes and ketones enolize to a sufficient extent to allow the reaction to occur. However, carboxylic acids, can be brominated in the alpha position with a mixture of Br₂ and PBr₃ in a reaction called the Hell-Volhard-Zelinskii reaction.

The mechanism of this reaction involves an acid bromide enol instead of the expected carboxylic acid enol. The reaction stats with the reaction of the carboxylic acid with PBr₃ to form the acid bromide and HBr. The HBr then catalyzes the formation of the acid bromide enol which subsequently reacts with Br₂ to give alpha bromination. Lastly, the acid bromide reacts with water to reform the carboxylic acid.