Phenol from a diazonium salt: practice and mechanism
First NaNO₂/acid at 0 °C turns the amino group into a diazonium salt (Ar–N₂⁺). N₂ is an excellent leaving group. On warming in water, water replaces the N₂ group and a phenol forms.
Example
On heating in water the diazonium salt loses N₂ and the very reactive aryl cation captures water, giving the phenol. That is why diazonium salts must be kept cold (0–5 °C) for other reactions.
Mechanism step by step
- 1
NaNO₂ and acid form the nitrosyl cation (NO⁺) at 0 °C. The amine attacks it; after proton shifts and loss of water the aryl diazonium ion Ar–N≡N⁺ forms.
- 2
On warming N₂ leaves; a very reactive aryl cation forms.
- 3
Water attacks the cation and after deprotonation the phenol forms.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Sandmeyer reaction (chloride) · Aryl iodide from a diazonium salt
Frequently asked questions
Why the detour via the diazonium salt?
It puts substituents where direct SEAr cannot: nitrate, reduce to the amine (a good director), then swap the amino group for Cl, Br, CN, OH, I, F or H.
Why sulfuric acid instead of hydrochloric acid?
With HCl, chloride would compete with water for the cation and give the aryl chloride. Hydrogen sulfate is a very weak nucleophile.