Preparing amines and diazonium dyesEdexcel International A Level Chemistry: Revision notes

A halogenoalkane reacts with excess concentrated ammonia in ethanol, heated in a sealed tube (under pressure), by nucleophilic substitution:

CH₃CH₂CH₂Br + 2NH₃ → CH₃CH₂CH₂NH₂ + NH₄Br

The first NH₃ is the nucleophile, the second takes the H⁺ from the ammonium ion formed. A large excess of ammonia makes it far more likely that a halogenoalkane molecule meets NH₃ than an amine molecule, which reduces further substitution to secondary and tertiary amines and quaternary salts. Some mixture still forms, so yields are modest.

Heating a halogenoalkane under reflux with KCN in ethanol gives a nitrile with one more carbon in the chain:

CH₃CH₂CH₂Br + CN⁻ → CH₃CH₂CH₂CN + Br⁻

The nitrile is then reduced to a primary amine with LiAlH₄ in dry ether, or with hydrogen gas and a nickel catalyst at elevated temperature and pressure:

CH₃CH₂CH₂CN + 4[H] → CH₃CH₂CH₂CH₂NH₂

This route gives a cleaner product than ammonia substitution and makes a longer chain.

Aromatic amines are made by reducing nitro compounds. Nitrobenzene is heated under reflux with tin and concentrated hydrochloric acid:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

The acid protonates the amine, so the flask contains phenylammonium ions, C₆H₅NH₃⁺. Excess sodium hydroxide is then added to release free phenylamine, which is separated (for example by steam distillation or solvent extraction).

Worked example: 12.3 g of nitrobenzene (Mr = 123) is 0.100 mol. At 100% yield this gives 0.100 mol of phenylamine (Mr = 93), or 9.30 g. At 60% yield the mass is 9.30 × 0.60 = 5.58 g.

Phenylamine reacts with nitrous acid, HNO₂, to form the benzenediazonium ion, C₆H₅N₂⁺ (C₆H₅–N⁺≡N). Nitrous acid is unstable so it is made in situ from sodium nitrite and hydrochloric acid, and the temperature is held below 10 °C (ice bath):

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

Above 10 °C the diazonium ion decomposes to phenol and nitrogen gas. The solution is used at once.

The diazonium ion is a weak electrophile. It attacks the ring of phenol in alkaline solution, where phenol exists as the more reactive phenoxide ion. The product, substituted at the 4-position, is 4-hydroxyazobenzene, a yellow-orange azo dye:

C₆H₅N₂⁺ + C₆H₅OH → HOC₆H₄N=NC₆H₅ + H⁺

The mixture is kept below 10 °C throughout.

In an azo dye the –N=N– group links the two rings, so the delocalised π system extends over both rings and the azo group. A larger delocalised system has a smaller energy gap between electronic energy levels, so electrons absorb visible light instead of ultraviolet. The colour seen is made of the visible wavelengths that are not absorbed. Changing the groups on the rings alters the extent of delocalisation, and so the colour of the dye.