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.