Diazonium salts

Making a diazonium salt. Treat an aromatic primary amine with sodium nitrite and hydrochloric acid at 273-278 K (0-5 °C): aniline gives benzenediazonium chloride, C₆H₅N₂⁺Cl⁻. This is diazotisation. It must be kept cold: diazonium salts are unstable and break up when warmed, so they are used straight away, in solution.

Aromatic diazonium salts are more stable than aliphatic ones because the positive charge is shared with the benzene ring by resonance.

Their great value is that N₂ is an excellent leaving group: it escapes as nitrogen gas, and almost anything can take its place on the ring. This lets us put groups on a ring that cannot be added directly.

• Sandmeyer reaction: CuCl/HCl gives Ar–Cl; CuBr/HBr gives Ar–Br; CuCN/KCN gives Ar–CN. (The Gattermann reaction does the first two with copper powder and HX.)
• Iodine: simply add KI: Ar–I.
• Fluorine: add fluoroboric acid (HBF₄), then heat the precipitate: Ar–F (the Balz-Schiemann reaction).
• –OH: warm with water: phenol.
• –H: treat with hypophosphorous acid (H₃PO₂) or ethanol: the group is simply removed, leaving the arene.
• –NO₂: the fluoroborate heated with NaNO₂ and copper gives a nitroarene.

Coupling reactions. Here the nitrogen stays. A diazonium salt reacts with phenol (in alkaline solution) or with aniline (in mildly acidic solution), joining at the para position through an –N=N– link. The products, azo compounds, have extended conjugated systems and are brightly coloured: orange, yellow, red. They are widely used as dyes.