Classification and basicity
Amines are derivatives of ammonia, NH₃, in which hydrogen atoms are replaced by alkyl or aryl groups. A primary amine has one group (CH₃NH₂), a secondary amine two ((CH₃)₂NH) and a tertiary amine three ((CH₃)₃N). The nitrogen has a lone pair, so amines are bases and nucleophiles. Primary aliphatic amines are stronger bases than ammonia because alkyl groups push electrons towards nitrogen; phenylamine (aniline) is a weaker base because the lone pair is delocalised into the benzene ring.
| Base | pK_b (approx.) | Strength |
|---|---|---|
| Dimethylamine | 3.3 | Strongest |
| Methylamine | 3.4 | Strong |
| Ammonia | 4.8 | Medium |
| Phenylamine | 9.4 | Weakest |
Preparation and reactions
| Preparation | Reagents |
|---|---|
| From a halogenoalkane | Excess ammonia in ethanol, heat under pressure: RBr + 2NH₃ → RNH₂ + NH₄Br |
| Reduction of a nitrile | LiAlH₄ or H₂ and nickel: RCN → RCH₂NH₂ |
| Reduction of nitrobenzene (phenylamine) | Tin and concentrated hydrochloric acid, then NaOH |
| Reaction | Description |
|---|---|
| With acids | Form ammonium salts: CH₃NH₂ + HCl → CH₃NH₃⁺Cl⁻ |
| With acyl chlorides | Form amides: CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl |
| With halogenoalkanes | Further substitution gives secondary and tertiary amines and quaternary salts |
| With copper(II) ions | Deep blue complex, like ammonia |
Diazonium salts and dyes
Phenylamine reacts with nitrous acid (NaNO₂ + HCl) below 5 °C to make a diazonium salt, C₆H₅N₂⁺Cl⁻. The salt is unstable above 10 °C. It couples with phenol or another aromatic compound to make azo dyes, which contain the –N=N– group and are brightly coloured. Azo dyes colour textiles, foods and indicators such as methyl orange.
Choose the concentration and pK_b of a weak base. For a weak base, pOH ≈ ½(pK_b − log c) and pH = 14 − pOH.
Key terms
- Amine
- A compound derived from ammonia by replacing hydrogen atoms with alkyl or aryl groups.
- Diazonium salt
- A compound containing the –N₂⁺ group, made from a primary aromatic amine and nitrous acid.
- Azo dye
- A dye containing the –N=N– group.
- Nucleophile
- A species that donates a lone pair.
Summary
- Amines are basic and nucleophilic; alkylamines are stronger bases than ammonia, phenylamine is weaker.
- Prepared from halogenoalkanes, nitriles or nitrobenzene.
- They make salts with acids and amides with acyl chlorides.
- Phenylamine gives diazonium salts below 5 °C, used to make azo dyes.