Hell-Volhard-Zelinsky reaction is the substitution organic reaction which forms alpha halo carboxylic acid from a carboxylic acid having alpha hydrogen in presence of phosphorous halide or phosphorous as a catalyst. The reaction has been named after 3 chemists who discovered this reaction in a period of 1881-1887. The three chemists were Carl Magnus von Hell, Jacob Volhard, Nikolay Zelinsky.
Mechanism: Firstly carbonyl oxygen reacts with phosphorous trihalide which leads to the formation of P-O bond and halide anion get released. Halide anion attacks carbonyl group which forms intermediate that undergoes rearrangement and acyl chloride, acid molecule and phosphine oxide are released. Further tautomerization of acyl chloride occurs which forms alpha halo acyl halide. Finally, hydrolysis takes place which leads to the formation of alpha halo carboxylic acid.