Sandmeyer Reaction
A radical mechanism, catalytic for copper(I) salt has been proposed for the Sandmeyer reaction, which converts an aryldiazonium halide/salt to an aryl halide or an aryl pseudohalide. The required aryldiazonium halide is usually prepared from arylamines (via diazotization) using either NaNO2/hydrohalic acid or alkyl nitrites under anhydrous conditions. A single electron-transfer (SET) from the copper(I) catalyst to the diazonium, creates a diazonium radical and copper(II) halide. The diazonium radical readily releases nitrogen gas, forming an aryl radical, which then receives a halide from copper(II) halide, regenerating the copper(I)catalyst and forming the final aryl halide.
- Reagents: HX, NaNO3, Catalytic copper
- Reactant: Aryl Diazonium Salt
- Product: Aryl Halide
Lab Tips
- Both electron-donating and electron-withdrawing groups are tolerated on the aryldiazonium halide.
- If the NaNO2/hydrohalic acid route is chosen to synthesize the aryldiazonium halide, the counterion of the copper(I) salt must match the conjugate base of the hydrohalic acid, otherwise a mixture of aryl halide product is to be expected. [1][1][1]
- Note that phenols can also be made from aryldiazonium salts by heating with trifluoroacetic acid, aqueous sulfuric acid, or with an aqueous solution of copper salts (occasionally referred to as the Sandmeyer hydroxylation). [2,3,4]
2. Horning, D. E., Ross, D. A., Muchowski, J. M. Synthesis of phenols from diazonium tetrafluoroborates. Useful modification. Can. J. Chem. 1973, 51, 2347-2348.
3. Cohen, T., Dietz, A. G., Jr., Miser, J. R. A simple preparation of phenols from diazonium ions via the generation and oxidation of aryl radicals by copper salts. J. Org. Chem. 1977, 42, 2053-2058.
4. Hanson, P., Rowell, S. C., Walton, P. H., Timms, A. W. Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands. Org. Biomol. Chem. 2004, 2, 1838-1855.
Mechanism
Original Paper
Top Citations
Related Reactions
- Azo Coupling
- Diazotisation
- Rosenmund-von Braun Reaction
- Schiemann Reaction
