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of phosphorous, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which α-hydrogen has been replaced by halogen.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

In the presence of a small amount of phosphorous, aliphatic carboxylic acids react with chlorine or bromine to yield a compound in which alpha-hydrogen has been replaced by halogen.This reaction is known as Hell - Volhard - Zelinsky reaction.
RCH₂COOH
i. P/Cl₂ →
ii. H₂O
RCH(Cl)COOH + HCl