Carboxylic acids and derivatives

Acyl chlorides and acylation

Acyl chlorides (RCOCl) are made from carboxylic acids with SOCl₂, PCl₅ or PCl₃. The carbonyl carbon is very δ+ (bonded to O and Cl), so they react vigorously at room temperature with nucleophiles by nucleophilic addition–elimination, giving off steamy HCl fumes: water → carboxylic acid; alcohol (or phenol) → ester; ammonia → primary amide; primary amine → N-substituted amide. Acid anhydrides ((RCO)₂O) react the same way but more slowly, releasing RCOOH instead of HCl; ethanoic anhydride is preferred industrially for aspirin (cheaper, safer, less corrosive). Ease of hydrolysis: acyl chloride ≫ alkyl chloride ≫ aryl chloride.