Acid Halide Formation

Thionyl chloride can be used to convert a carboxylic acid into an acid chloride. (Phosphorus tribromide will accomplish an analogous reaction, converting carboxylic acids to acid bromides.) The mechanism is composed of two successive nucleophilic acyl substitutions, the first substitution converting the carboxylic acid into the reactive chlorosulfite form, which is then attacked by chlorine anion, resulting in the formation of the acid chloride product.