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.
15.10:
α-Halogenation of Carboxylic Acid Derivatives: Overview
Unlike aldehydes and ketones, carboxylic acids do not readily participate in α halogenation reactions via enols or enolate intermediates. However, α-halogenated acids are obtained through other methods. One of the approaches is the Hell–Volhard–Zelinsky (HVZ) reaction, wherein the carboxylic acid is treated with halogen in the presence of PBr3. It involves the conversion of acid to acid halide, which exists in equilibrium with its enol form. The enol attacks the electrophilic halogen to produce an α-haloacid halide, which gives the α-halogenated acid upon hydrolysis. Other methods include the formation of acid chloride using sulfonyl chloride, which, upon treatment with N-halosuccinimide and traces of hydrogen halide, forms α-haloacid halide, and final hydrolysis gives the desired product.