Nucleophilic Addition-Elimination Mechanism

Acyl chlorides and acid anhydrides react with nucleophiles by nucleophilic addition-elimination. AQA requires this mechanism for the reactions of acyl chlorides with water, alcohols, ammonia and primary amines. The example below is ethanoyl chloride with methanol.Step 1: addition. A lone pair on the oxygen of methanol attacks the δ⁺ carbonyl carbon. At the same time the electrons of the C=O π bond move onto the oxygen atom. This forms an intermediate in which the carbon has four groups attached, the old carbonyl oxygen carries a negative charge, and the methanol oxygen carries a positive charge.Step 2: elimination. A lone pair on the negative oxygen moves back to reform the C=O bond, and the C-Cl bond breaks, so a chloride ion leaves.Step 3: loss of H⁺. The positively charged oxygen loses an H⁺ ion (the O-H bond electrons move onto the oxygen). The H⁺ combines with Cl⁻ to form HCl.CH₃COCl + CH₃OH → CH₃COOCH₃ + HClThe same pattern works for every nucleophile. With water the attacking atom is oxygen; with ammonia or a primary amine it is nitrogen, so the intermediate carries its positive charge on nitrogen. For ammonia and amines, the H⁺ is often shown being removed by a second molecule of ammonia or amine, which is why two moles are used. When drawing the mechanism, curly arrows must start from a lone pair or a bond and point to where the electrons go.