EFFECT OF MANGANESE(2) IONS ON OXIDATION OF MANDELIC ACID BY AQUEOUS CHROMIC ACID

The oxidation of mandelic acid by aqueous chromic acid is accelerated by the addition of manganese(II) sulphate if the [MnII] : [Cr VI] ratio is over 100 : 1, but at lower concentrations Mn II is a retarder of the oxidation. CrIII does not diminish the oxidation rate which, when catalysed, approaches zero order with respect to [CrVI]. The discussion suggests that the two equilibria Cr VI + MnII ⇌ CrV + MnIII and CrIV + MnII ⇌ CrIII + MnIII are involved in the reaction process and that one-electron reductions of both CrVI and CrV by the free radical Ph·CH(OH)· play an important role in the overall reaction.