Some metal-ion catalysed reactions of chromic acid

5 SOME METAL-ION CATALYSED REACTIONS OF CHROMIC ACID 

Dhar noted that the oxidation of oxalic acid by chromic acid is markedly accelerated on adding manganous ions, the reaction order in Cr(VI) changing from one to zero. Bobtelsky and Glasner ° found the oxidation of bromide ions by chromic acid in aqueous sulphuric acid to follow kinetics 

The slow step is (32), which excludes any participation by bromide in determining the overall rate. 

Kemp and Waters also found the oxidations of cyclohexanone and of mandelic, malonic and a-hydroxyisobutyric acids by Cr(VI) to be Mn(II)-catalysed. In these cases, as with oxalic acid, the [Cr(VI)] versus time plots are almost linear and the reaction becomes first order in substrate (or involves Michaelis-Menten kinetics), and, except at lowest catalyst concentrations, approximately first order in [Mn(II)]. Detailed examination of the initial rate of oxidation of a-hydroxyrobutyric acid as a function of oxidant concentration revealed, however, that the dependence is 

This can be explained in terms of the pre-equilibria (32)-(34) followed by a slow oxidation of the substrate by Mn(III). It is probable that the substrates are chelated to Mn(II) and Mn(III) throughout the process. The rate of oxidation of the substrate is given by 

---