33.1.2—Carboxylic acids to acyl chlorides

Carboxylic acids react with chlorinating agents such as PCl₃ with heat, PCl₅ or SOCl₂ to replace the hydroxyl group by chlorine, forming an acyl chloride.

The acyl chloride is more reactive because chloride is a better leaving group than hydroxide. It can then be used to make esters, amides or other acyl derivatives.

Ethanoic acid + PCl₅ → ethanoyl chloride plus phosphorus-containing products and HCl. The exact by-products depend on the reagent.

Do not treat the reaction as simple substitution at a saturated carbon; the carbonyl carbon is the electrophilic centre.