Overall reaction
C₆H₅NH₂ → (NaNO₂, HCl, 0–5 °C) C₆H₅N₂⁺Cl⁻ → (CuCl/HCl) C₆H₅Cl + N₂
Copper(I) replaces N₂⁺ by Cl, Br or CN through an aryl radical.
Name reaction · mechanism
Sandmeyer Reaction
The Sandmeyer reaction replaces the diazonium group of an arenediazonium salt by Cl, Br or CN using the copper(I) salt. Copper(I) gives one electron to the diazonium ion, nitrogen gas is lost to leave an aryl radical, and the radical takes the halogen from copper(II), regenerating copper(I). The diazonium salt is made first from aniline, NaNO₂ and HCl at 0–5 °C.
Mechanism step by step
- Electron transfer. Cu(I) gives one electron; N₂ leaves and an aryl radical forms
- Chlorine transfer. The radical takes Cl from Cu(II), regenerating Cu(I)
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The diazonium salt comes from aniline, NaNO₂ and HCl at 0–5 °C (see Diazotisation).
- CuCl → ArCl, CuBr → ArBr, CuCN → ArCN (a route to benzoic acids and benzylamines).
- Gattermann reaction: Cu powder with HCl/HBr does the same job.
- ArI needs no copper (KI); ArF uses HBF₄ (Balz–Schiemann).
- Biaryl by-products (Ar–Ar) are evidence for the radical mechanism.
For JEE and NEET
- Sandmeyer uses Cu(I) salts; the Gattermann reaction uses copper powder with HCl or HBr.
- ArI is made with KI alone (no copper); ArF by the Balz–Schiemann reaction (HBF₄, heat).
- Diazonium salts must be kept at 0–5 °C; warming in water gives phenol.
- H₃PO₂ (or ethanol) replaces N₂⁺ by H (deamination).
FAQ
What is the role of copper in the Sandmeyer reaction?
Copper(I) transfers one electron to the diazonium ion; after N₂ leaves, the aryl radical takes Cl, Br or CN from copper(II), so copper(I) is regenerated (catalytic in principle).
What is the difference between Sandmeyer and Gattermann reactions?
Both convert diazonium salts to aryl halides. Sandmeyer uses CuCl/CuBr; Gattermann uses copper powder and the halogen acid, usually in lower yield.
Why is diazotisation done at 0–5 °C?
Arenediazonium salts decompose on warming, losing N₂ and reacting with water to give phenols.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.