Sandmeyer reaction (chloride): 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. CuCl replaces the N₂ group by Cl, exactly where the NH₂ used to be.
Example
Nitrous acid turns the aniline into an aryl diazonium salt (Ar–N₂⁺). Copper(I) chloride transfers an electron, N₂ escapes and the aryl radical takes chlorine from the copper. NH₂ is thus replaced by Cl.
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 to the diazonium ion; N₂ escapes and an aryl radical remains.
- 3
The aryl radical takes a Cl from CuCl₂; the aryl chloride forms and Cu(I) is regenerated.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
First the diazonium salt
An aniline is diazotised at 0–5 °C with sodium nitrite (NaNO₂) and hydrochloric acid. The aryl diazonium salt is only stable in the cold and is used straight away. N₂ is an excellent leaving group, so the amino group can be swapped for many other groups this way.
The Sandmeyer reactions
With copper(I) salts: CuCl gives aryl chlorides, CuBr aryl bromides, CuCN aryl nitriles. The mechanism runs through a single-electron transfer from copper and an aryl radical. Iodides form without copper using KI, fluorides via the tetrafluoroborate (Balz–Schiemann), phenols by warming in water.
Why it is so useful in synthesis
The amino group directs a second substitution ortho/para and can be swapped out afterwards. This gives substitution patterns that direct electrophilic substitution cannot reach, for example 1,3,5-tribromobenzene via tribromoaniline.
Typical mistakes
Often confused with: Sandmeyer reaction (bromide) · Sandmeyer reaction (nitrile) · Phenol 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.
What is the Sandmeyer reaction?
Replacing the diazonium group of an aryl diazonium salt with Cl, Br or CN using copper(I) salts (CuCl, CuBr, CuCN). The diazonium salt is made first from an aniline with NaNO₂ and HCl at 0–5 °C.