8.0 Hell-Volhard-Zelinsky Reaction

  Carboxylic Acids and Derivatives

This is $\alpha $-halogenation of a carboxylic acid.

Possible Mechanism for the Reaction is

The purpose of $PB{r_3}$ is to convert —OH into —Br to make $\alpha $-hydrogen atom more acidic to be replaced by Br atom of $B{r_2}$. The reaction does not stop at monosubstitution but continues till the $\alpha $-hydrogen are replaced.

The reaction has a great synthetic importance as the halogen atom can be replaced by a number of other groups giving useful products.