Carboxylic acids react with SOCl₂ or PCl₅ to form acid chlorides. Amongst the carboxylic acid derivatives, acid chlorides are the most reactive and synthetically important derivatives. They are useful reagents for Friedel–Crafts acylation of some aromatic compounds.

An alternative reagent for converting a carboxylic acid to an acid chloride is phosphorus pentachloride. The mechanism involves the attack by a carboxylic acid at the phosphorus center of PCl₅ while eliminating a chloride ion. Subsequently, the attack by this chloride ion at the carbonyl center results in an acid chloride, along with the formation of phosphorus oxychloride. The formation of a highly stable P=O bond drives the reaction forward.