Amines as Nucleophiles

A nucleophile is an electron pair donor. Amines have a lone pair on the nitrogen, so they attack δ⁺ carbon atoms. AQA covers two types of reaction.1. With halogenoalkanes: nucleophilic substitution. The amine's lone pair attacks the δ⁺ carbon of the C-X bond and the halide ion leaves. A primary amine therefore becomes a secondary amine:CH₃CH₂NH₂ + CH₃CH₂Br → (CH₃CH₂)₂NH + HBrThe product is diethylamine (the HBr reacts with more amine to form a salt). The secondary amine still has a lone pair, so substitution can continue:primary → secondary → tertiary amine → quaternary ammonium saltThe final product, R₄N⁺X⁻, has no lone pair, so the chain stops there. The ratio of reactants controls the outcome: an excess of the amine favours the first substitution, while an excess of the halogenoalkane pushes the reaction all the way to the quaternary ammonium salt.2. With acyl chlorides and acid anhydrides: nucleophilic addition-elimination. The amine attacks the carbonyl carbon and the product is an N-substituted amide:CH₃COCl + 2CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + CH₃CH₂NH₃⁺Cl⁻Worked example: the product of ethylamine and ethanoyl chloride is CH₃CONHCH₂CH₃. The acyl part (2 carbons) gives ethanamide, and the ethyl group on the nitrogen gives the prefix N-ethyl: N-ethylethanamide.