A study of the mechanism of the Sandmeyer reaction

The one-electron-transfer mechanism for the Sandmeyer reaction, proposed by W. A. Waters in 1942 has now been confirmed by diagnostic experiments. Its initial stage, Ar ·N + 2 + Cu + → Ar · + N 2 + Cu 2+ , has been shown to produce free aryl radicals capable of initiating vinyl polymerization; this observation is also true for related reactions of diazonium salts for which analogous one-electron-transfer mechanisms have been suggested. The cupric ions thus produced are oxidizers that, like ferric ions, can effect chain-ending of vinyl polymerization. They bring about the final step in the Sandmeyer reaction by a similar process that apparently involves chlorine atom transfer; Ar · + Cl—Cu—Cl → Ar —Cl + Cu—Cl. Product studies show that cupric ions are needed to enable the Sandmeyer reaction to give aryl chlorides in high yield; in their absence other reactions of aryl radicals can occur extensively. Molecular weight measurements with acrylonitrile polymers indicate that in dilute solution up to 35% of the initial aryldiazonium cations can produce free aryl radicals by interaction with cuprous ions in chloride solutions, but that in more concentrated solutions the secondary reaction with the cupric chloride generated occurs so extensively that the full cycle of the Sandmeyer reaction becomes predominantly an intramolecular process.