Amines: properties and reactionsEdexcel International A Level Chemistry: Revision notes

Primary amines contain –NH₂ and are named as alkanamines: CH₃CH₂CH₂CH₂NH₂ is butan-1-amine (butylamine). The simplest aromatic amine, C₆H₅NH₂, is phenylamine. Substituents on nitrogen are shown with N-: CH₃NHCH₂CH₃ is N-methylethanamine (a secondary amine).

Amides contain –CONH₂ and end in -amide: CH₃CONH₂ is ethanamide, and CH₃CONHCH₃ is N-methylethanamide (an N-substituted amide). Amino acids are named as substituted carboxylic acids: H₂NCH₂COOH is 2-aminoethanoic acid (glycine) and CH₃CH(NH₂)COOH is 2-aminopropanoic acid (alanine).

In displayed formulae every bond is drawn. In skeletal formulae carbon atoms are the corners and line ends, hydrogens on carbon are omitted, but N, O and the H atoms on N or O are written, e.g. a zig-zag chain ending in NH₂.

The nitrogen in an amine has a lone pair, so amines are proton acceptors (Brønsted–Lowry bases). In water a small amount of hydroxide forms, so the solution is alkaline:

CH₃CH₂CH₂CH₂NH₂ + H₂O ⇌ CH₃CH₂CH₂CH₂NH₃⁺ + OH⁻

Phenylamine reacts in the same way but the equilibrium lies even further left. With acids a salt forms, which is ionic and water soluble:

CH₃CH₂CH₂CH₂NH₂ + HCl → CH₃CH₂CH₂CH₂NH₃⁺Cl⁻

C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻ (phenylammonium chloride)

Adding sodium hydroxide to the salt removes the proton and releases the free amine.

Amines have an N–H bond and a lone pair on nitrogen, so they form hydrogen bonds with water. The δ+ hydrogen of water is attracted to the lone pair on N, and the δ+ hydrogen of N–H is attracted to the lone pair on oxygen. Short-chain amines such as butylamine are therefore miscible with water. Longer carbon chains and the benzene ring in phenylamine increase the non-polar part of the molecule, so solubility falls.

Basicity depends on how available the nitrogen lone pair is to accept H⁺.

• Primary aliphatic amines (e.g. butylamine) are stronger bases than ammonia. The alkyl group is electron-releasing (positive inductive effect), so electron density on N increases.
• Ammonia has no electron-releasing group.
• Phenylamine is weaker than ammonia. The nitrogen lone pair overlaps with and is delocalised into the π system of the benzene ring, so it is less available.

Order of basicity: butylamine > ammonia > phenylamine.

Because of the lone pair, amines are nucleophiles.

With halogenoalkanes (nucleophilic substitution): the lone pair attacks the δ+ carbon and the halide leaves. CH₃CH₂CH₂CH₂NH₂ + CH₃CH₂Br → CH₃CH₂CH₂CH₂NHCH₂CH₃ + HBr (as the salt). The product is a secondary amine which still has a lone pair, so reaction can continue to a tertiary amine and a quaternary ammonium salt.

With ethanoyl chloride (nucleophilic addition–elimination): CH₃COCl + CH₃CH₂CH₂CH₂NH₂ → CH₃CONHCH₂CH₂CH₂CH₃ + HCl. The product is an N-substituted amide, N-butylethanamide. Phenylamine gives N-phenylethanamide, C₆H₅NHCOCH₃, in the same way. The HCl formed reacts with more amine to give an ammonium chloride salt.

Aqueous butylamine added to copper(II) sulfate gives a pale blue precipitate of copper(II) hydroxide, as the amine produces OH⁻ in water. In excess amine the precipitate dissolves to a deep blue solution. The amine is a ligand: its nitrogen lone pair forms a dative covalent bond to Cu²⁺, replacing water ligands (ligand substitution), e.g. [Cu(H₂O)₂(RNH₂)₄]²⁺. This is the same behaviour as ammonia.