Nitrogen Compounds (A Level)

• Amines are made from halogenoalkanes with NH₃ or amines, and by reducing amides or nitriles.
• Amines react with acyl chlorides to give amides; their aqueous solutions are basic.
• Phenylamine is made from nitrobenzene; it reacts with Br₂(aq) and with HNO₂ to form a diazonium salt.
• Base strength: ethylamine > ammonia > phenylamine.
• Diazonium salts couple with phenol to form azo dyes.
• Amides are hydrolysed by acid or alkali, reduced by LiAlH₄, and are much weaker bases than amines.
• Amino acids form zwitterions, link through peptide bonds and separate by electrophoresis.

Making amines

The halogenoalkane route gives a mixture: the amine product is itself a nucleophile and can react again to give secondary and tertiary amines and a quaternary ammonium salt. A large excess of ammonia favours the primary amine. The HBr formed reacts with excess NH₃ or amine to give a salt.

Amines with acyl chlorides

Ammonia or an amine reacts with an acyl chloride at room temperature in a condensation reaction to give an amide and HCl: CH₃COCl + CH₃CH₂NH₂ → CH₃CONHCH₂CH₃ + HCl.

Basicity

The lone pair on N accepts a proton, so aqueous amines are alkaline: CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻. They are weak bases (the equilibrium lies to the left). With acids they form salts: CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻.

Making phenylamine

1. Nitrate benzene (conc HNO₃ + conc H₂SO₄, 25–60 °C) to nitrobenzene.
2. Reduce nitrobenzene by heating with tin and concentrated HCl: C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O. In the acid the product is present as the salt C₆H₅NH₃⁺.
3. Add NaOH(aq) to release the free amine: C₆H₅NH₃⁺ + OH⁻ → C₆H₅NH₂ + H₂O.

Reactions of phenylamine

• Br₂(aq), room temperature: decolourised, white precipitate of 2,4,6-tribromophenylamine. C₆H₅NH₂ + 3Br₂ → C₆H₂Br₃NH₂ + 3HBr. As with phenol, the N lone pair is delocalised into the ring, activating it.
• HNO₂ (NaNO₂ + dilute HCl) below 10 °C: benzenediazonium chloride. C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O. Warming this with water gives phenol: C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl.

Basicity: ammonia, ethylamine and phenylamine

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