Acyl chlorides — common mistakes

Examiners frequently ask why two equivalents of amine or ammonia are needed. The first equivalent acts as the nucleophile to attack the carbonyl carbon. The second equivalent acts as a base to neutralise the HCl by-product, forming an ammonium salt. If you only use one equivalent, half of your expensive amine/ammonia will be protonated and become unreactive, leading to a maximum 50% yield.

Acyl chlorides react very rapidly with water vapour present in the air in a hydrolysis reaction. This reaction produces the parent carboxylic acid and hydrogen chloride (HCl) gas. The HCl gas then appears as white, steamy fumes.

The first mole of the amine acts as a nucleophile, attacking the carbonyl carbon to form the amide. This reaction also produces one mole of HCl as a by-product. Since amines are bases, a second mole of the amine is required to react with and neutralise the acidic HCl, forming an ammonium salt (e.g., $RNH_3Cl$).

No, you cannot make a tertiary amide (an amide where the nitrogen is bonded to three carbon atoms) using an acyl chloride and a tertiary amine. While a tertiary amine can act as a catalyst, it cannot form a stable amide product because it lacks an N-H bond. The final step of the mechanism involves the removal of a proton from the nitrogen atom, which is not possible for a tertiary amine.