Overall reaction
C₆H₅NH₂ + NaNO₂ + 2 HCl (0–5 °C) → C₆H₅N₂⁺Cl⁻ + NaCl + 2 H₂O
Aniline + nitrous acid at 0–5 °C: the arenediazonium ion.
Name reaction · mechanism
Diazotization Reaction
Diazotization converts a primary aromatic amine into an arenediazonium salt with sodium nitrite and a mineral acid at 0–5 °C. Nitrous acid forms the nitrosonium ion (NO⁺), the amine attacks it, and a series of proton transfers and loss of water give Ar–N⁺≡N.
Mechanism step by step
- Protonation. HNO₂ (from NaNO₂ + HCl) is protonated
- Nitrosonium ion. Water leaves: N≡O⁺
- N-Nitrosation. The amine attacks NO⁺
- Deprotonation. The N-nitrosamine
- O-Protonation. Tautomerisation begins
- Diazohydroxide. Loss of the N–H proton gives N=N
- Protonation. OH becomes a leaving group
- Diazonium ion. Water leaves: Ar–N⁺≡N
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
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.
For JEE and NEET
- Electrophile: the nitrosonium ion NO⁺.
- Aliphatic primary amines give unstable diazonium ions that lose N₂ at once (alcohols, alkenes).
- Diazonium salts: Sandmeyer (CuX), KI, H₂O/heat (phenol), HBF₄ (Balz–Schiemann), H₃PO₂ (ArH), azo coupling.
- Benzenediazonium is stabilised by resonance with the ring.
FAQ
Why must diazotization be done at 0–5 °C?
Arenediazonium salts lose N₂ when warmed, and the aryl cation reacts with water to give phenol.
What is the electrophile in diazotization?
The nitrosonium ion, N≡O⁺, from protonation of HNO₂ and loss of water.
Why are aryl diazonium salts more stable than alkyl ones?
The C–N bond to an sp² ring carbon is stronger, an aryl cation is very unstable, and the N₂⁺ group is conjugated with the ring.
Draw it in MolDraw
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