15.10 : α-Halogenation of Carboxylic Acid Derivatives: Overview

Unlike α-halogenation of aldehydes and ketones that occurs through enol or enolate intermediates, carboxylic acids and their derivatives do not readily undergo such reactions due to inadequate enolization.

This is because such molecules, under basic conditions, undergo deprotonation of the more acidic hydrogen, thereby inhibiting enolate formation.

α-halogenated acids are obtained by the conversion of the corresponding acids into their acyl halide derivatives.

Acid halides exist in equilibrium with their enol forms. Treating these with suitable halogenating reagents, followed by hydrolysis, gives the desired product.

One of the methods to achieve this is the Hell–Volhard–Zelinsky reaction, which uses phosphorus halide and a halogen to give α-halogenated acid.

In another approach, carboxylic acids react with thionyl chloride to generate acyl chloride. Concomitant reaction with a suitable  N-halosuccinimide in traces of hydrogen halide and subsequent hydrolysis yields the desired product.