Sandmeyer reaction (bromide): 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. CuBr replaces the N₂ group by Br at the position of the former amino group.
Example
Via the diazonium salt the amino group is exchanged for bromine; CuBr mediates the exchange by a radical pathway. Advantage over direct bromination: the bromine lands exactly where the NH₂ was.
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
CuBr donates an electron, N₂ escapes and an aryl radical forms.
- 3
The aryl radical takes a Br from copper(II) bromide: a bromobenzene derivative.
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 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.
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.