Diazonium salts

The essentials

Diazonium salts have the general formula ArN₂⁺X⁻, for example benzenediazonium chloride, C₆H₅N₂⁺Cl⁻. They are made by diazotisation: treating an aromatic primary amine with nitrous acid (formed from NaNO₂ and HCl) at 273–278 K. Arenediazonium salts are stabilised by resonance of the –N₂⁺ group with the ring, but they are still used immediately in solution because they decompose on warming. Aliphatic diazonium salts are highly unstable. Benzenediazonium fluoroborate is stable at room temperature and insoluble in water.

Key points

• Replacement by halide or cyanide: CuCl/HCl, CuBr/HBr or CuCN give chloro-, bromo- or cyanobenzene (Sandmeyer reaction). Copper powder with HCl or HBr also works (Gattermann reaction).
• KI gives iodobenzene (no copper needed).
• HBF₄ gives the fluoroborate, which on heating gives fluorobenzene.
• H₃PO₂ or ethanol replaces –N₂⁺ by H, giving benzene.
• Warm water replaces –N₂⁺ by –OH, giving phenol.
• Fluoroborate heated with NaNO₂ and Cu gives nitrobenzene.
• Coupling reactions (N₂ retained): with phenol in alkaline medium → p-hydroxyazobenzene (orange); with aniline in mildly acidic medium → p-aminoazobenzene (yellow). These azo compounds are dyes.
• Importance: diazonium salts allow groups such as –F, –I, –CN and –OH to be put on a benzene ring, which is hard to do by direct substitution; they also help to make substitution patterns (e.g. 1,3,5-tribromobenzene from aniline).

Exam tips

• Remember the temperature 273–278 K and the colours of the coupling products.

Common mistakes

• Mixing up Sandmeyer (copper(I) salts) and Gattermann (copper powder with halogen acid).