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.