Diazotisation template

Nitrous acid gives the nitrosonium ion, aniline attacks it, tautomerisation gives the diazohydroxide and loss of water gives benzenediazonium.

Mechanism step by step

Aniline + nitrous acid at 0–5 °C: the arenediazonium ion

1. Protonation – HNO₂ (from NaNO₂ + HCl) is protonated
2. Nitrosonium ion – Water leaves: N≡O⁺
3. N-Nitrosation – The amine attacks NO⁺
4. Deprotonation – The N-nitrosamine
5. O-Protonation – Tautomerisation begins
6. Diazohydroxide – Loss of the N–H proton gives N=N
7. Protonation – OH becomes a leaving group
8. Diazonium ion – Water leaves: Ar–N⁺≡N

Key points

• Kept at 0–5 °C: above this the diazonium salt loses N₂ and gives phenol.
• Aryl diazonium ions are stabilised by the ring; alkyl ones decompose at once.
• Products: Sandmeyer (CuCl, CuBr, CuCN), KI → ArI, H₂O/heat → ArOH, HBF₄ → ArF (Balz–Schiemann), H₃PO₂ → ArH.
• Azo coupling with phenols or anilines gives azo dyes.

Drawn with 152 atoms, 26 arrows, 10 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook