Hell-Volhard-Zelinsky reaction

Schematic of the Hell-Volhard-Zelinsky reaction. Reagents: carboxylic acid with alpha-hydrogen, halogen gas, phosphorus catalyst, water. Product: alpha-halo carboxylic acid.

The Hell-Volhard-Zelinsky reaction is an organic reaction used to convert a carboxylic acid with an α-hydrogen and a halogen, to an α-halo carboxylic acid, using a phosphorous catalyst and water. The mechanism begins with the reaction of the carbonyl oxygen with phosphorous trihalide to form a P–O bond and release a halide anion. The halide then attacks the carbonyl to form an intermediate which rearranges to release an acyl chloride, an acid molecule, and a phosphine oxide. The acyl chloride then tautomerizes to the enol form which subsequently attacks the halogen molecule to form an α-halo acyl halide. Water hydrolysis yields the final α-halo carboxylic acid product.[1][2][3]


Mechanism of the Hell-Volhard-Zelinsky reaction. Step 2 is attack of a water nucleophile.


Hell, C. Ber. Dtsch. Chem. Ges. 1881, 14, 891–893.
Volhard, J. Justus Liebigs Ann. Chem. 1887, 242, 141–163.
Zelinsky, N. Ber. Dtsch. Chem. Ges. 1887, 20, 2026–2026.