Sandmeyer reaction template

CuCl₂⁻ gives one electron to benzenediazonium, which loses N₂ to a phenyl radical; the radical takes Cl from copper(II) and copper(I) is regenerated.

Mechanism step by step

Copper(I) replaces N₂⁺ by Cl, Br or CN through an aryl radical

1. Electron transfer – Cu(I) gives one electron; N₂ leaves and an aryl radical forms
2. Chlorine transfer – The radical takes Cl from Cu(II), regenerating Cu(I)

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.

Drawn with 57 atoms, 9 arrows, 4 shapes – every one editable in MolDraw.

Sources

• Sandmeyer, Ber. Dtsch. Chem. Ges. 1884, 17, 1633 Original paper
• Kochi, J. Am. Chem. Soc. 1957, 79, 2942 Original paper
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook