33.3.1—Preparation of acyl chlorides

Carboxylic acids are converted to acyl chlorides by PCl₃ with heat, PCl₅ or SOCl₂. The hydroxyl group is replaced by chlorine at the acyl carbon.

Choose the reagent named in the question and remember that each route has different inorganic by-products. The product is a reactive derivative ready for further acyl substitution.

Ethanoic acid + SOCl₂ → ethanoyl chloride; the reagent is convenient because gaseous by-products can leave the mixture.

Do not confuse an acyl chloride RCOCl with an alkyl chloride RCl; the carbonyl group changes its reactivity and naming.