Sandmeyer reaction (nitrile): 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. CuCN replaces the N₂ group by CN: a benzonitrile forms, with one extra carbon on the ring.
Example
With copper(I) cyanide the diazonium group is replaced by a nitrile group. This forms a new C–C bond on the ring; the nitrile can be turned further into a carboxylic acid or an amine.
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
Cu(I) transfers an electron, N₂ escapes and an aryl radical forms.
- 3
The radical takes the CN group from copper: benzonitrile.
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) · Sandmeyer reaction (bromide)
Frequently asked questions
Why work at 0 °C?
Aryl diazonium salts decompose on warming (N₂ escapes) and can even explode when dry. In the cold they stay stable in solution long enough for the second step.
What can be made from the benzonitrile?
Hydrolysis gives benzoic acid, reduction with LiAlH₄ benzylamine, a Grignard reagent an aryl ketone.