In the presence of a small amount of phosphorous, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which a-hydrogen has been replaced by halogen. The reaction is known as

The reaction is known as the Hell-Volhard-Zelinsky reaction.

Carboxylic acids are alpha-bromated in this process. In the presence of a little quantity of red phosphorous, carboxylic acids react with chlorine or bromine to produce alpha halo carboxylic acids.