Effect of manganese(II) ions on the oxidation of mandelic acid by aqueous chromic acid

Abstract

The oxidation of mandelic acid by aqueous chromic acid is accelerated by the addition of manganese(II) sulphate if the [Mn^II] : [Cr^VI] ratio is over 100 : 1, but at lower concentrations Mn^II is a retarder of the oxidation. Cr^III does not diminish the oxidation rate which, when catalysed, approaches zero order with respect to [Cr^VI].

The discussion suggests that the two equilibria Cr^VI+ Mn^II⇌ Cr^V+ Mn^III and Cr^IV+ Mn^II⇌ Cr^III+ Mn^III are involved in the reaction process and that one-electron reductions of both Cr^VI and Cr^V by the free radical Ph·CH(OH)· play an important role in the overall reaction.