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SubstitutionAdvanced
Sandmeyer Reaction
Also known as: Sandmeyer
Discovered by Traugott Sandmeyer (1884)
General Equation
ArNH₂ → ArN₂⁺ → ArX (via CuX)
Description
Conversion of an aryldiazonium salt to an aryl halide (Cl, Br) or nitrile (CN) using copper(I) salts. The diazonium salt is first prepared by treating the arylamine with NaNO₂/HCl at 0°C. Essential for introducing substituents not easily placed by direct EAS.
Reaction Scheme
Reactants:
- • Aryldiazonium salt (ArN₂⁺)
Products:
- • Aryl halide or nitrile (ArCl, ArBr, ArCN)
Reagents:
- • CuX (CuCl, CuBr, CuCN)
Conditions:
- • 0-5°C (diazotization), then warm
Mechanism (2 steps)
1
Diazotization: The arylamine reacts with NaNO₂/HCl at 0-5°C to form the diazonium salt (ArN₂⁺).
Arrow: curvedArN₂⁺ Cl⁻ (diazonium salt)
Must be kept cold — diazonium salts decompose above 5°C
2
The Cu(I) salt mediates replacement of N₂ with X (Cl, Br, or CN). N₂ gas escapes as an excellent leaving group.
Arrow: curved
Cu(I) is essential — simple halide doesn't work (except for I⁻)
Key Points to Remember
- ✓ArNH₂ → ArN₂⁺ → ArCl (CuCl), ArBr (CuBr), ArCN (CuCN)
- ✓ArN₂⁺ + KI → ArI (no Cu needed — direct substitution)
- ✓ArN₂⁺ + H₃PO₂ → ArH (reduction — removes NH₂ group)
- ✓ArN₂⁺ + HBF₄ → ArF (Balz-Schiemann reaction)
- ✓ArN₂⁺ + phenol → azo dye (diazo coupling)
- ✓NH₂ is obtained from NO₂ by reduction (Fe/HCl, SnCl₂, or H₂/Pd)
- ✓Key for placing substituents in positions not accessible by direct EAS
Examples
Reactant:
Aniline → Diazonium salt
Reagent:
NaNO₂/HCl (0°C), then CuCl
Product:
Chlorobenzene
Yield: 70%
Reactant:
p-Toluidine → Diazonium salt
Reagent:
NaNO₂/HBr (0°C), then CuBr
Product:
p-Bromotoluene
Yield: 75%
Reactant:
p-Nitroaniline → Diazonium salt
Reagent:
NaNO₂/HCl (0°C), then CuCN
Product:
p-Nitrobenzonitrile
Yield: 65%
Synthetic Utility
Enables EAS reactions that would otherwise be impossible. The -NH₂ group is a versatile synthetic handle that can be converted to many different substituents via the diazonium salt.
Limitations
- ⚠Diazonium salts are unstable and potentially explosive when dry
- ⚠Must keep at 0-5°C
- ⚠Yields can be moderate (60-80%)
- ⚠Aliphatic diazonium salts too unstable (only aromatic)
#diazonium#Sandmeyer#aromatic#amine to halide#Cu