How is benzenediazonium chloride prepared? Write its reactions with (i) CuCl/HCl (ii) KI (iii) H₃PO₂ (iv) warm water (v) phenol in alkaline solution.

1. 1 · Diazotisation. C₆H₅NH₂ + NaNO₂ + 2HCl → (273 to 278 K) C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O. This is diazotisation.
2. 2 · Say why it is kept cold. It is stable only in the cold (charge spread over the ring by resonance); it is used at once.
3. 3 · With CuCl/HCl. C₆H₅N₂⁺Cl⁻ → (CuCl/HCl) C₆H₅Cl + N₂: the Sandmeyer reaction.
4. 4 · With KI. C₆H₅N₂⁺Cl⁻ + KI → C₆H₅I + KCl + N₂: iodobenzene.
5. 5 · With H₃PO₂. C₆H₅N₂⁺Cl⁻ + H₃PO₂ + H₂O → C₆H₆ + N₂ + H₃PO₃ + HCl.
6. 6 · With warm water. C₆H₅N₂⁺Cl⁻ + H₂O → (283 K) C₆H₅OH + N₂ + HCl: phenol.
7. 7 · With phenol. C₆H₅N₂⁺Cl⁻ + C₆H₅OH + OH⁻ → p-HOC₆H₄N=NC₆H₅ + Cl⁻ + H₂O: coupling gives an orange azo dye, like those used on cotton cloth.
8. 8 · Close in one line. So one diazonium salt can be turned into many aromatic compounds and dyes.

Scoring tip: Write 273 to 278 K for diazotisation and the pH (mildly alkaline or acidic) for coupling; both are marks the examiner looks for.