Hell Volhard Zelinsky (HVZ) reaction:
This reaction involves alpha bromination of carboxylic acids. Carboxylic acids react with chlorine or bromine in presence of small amount of red phosphorous to give alpha halo carboxylic acids.
CH3−CH2−COOHBr2/P−−−−→CH3−CHBr−COOHBr2/P−−−−→CH3−CBr2−COOHBr2/P−−−−→ No further reaction.