Kinetics and mechanism of rhodium(III) catalysed oxidation of polyhydric alcohols by acidified potassium bromate

The kinetics of the RhIII catalysed oxidation of ethylene glycol and glycerol by an acidified solution of KBrO3 in the presence of Hg(OAc)2 as a scavenger, have been studied in the 30-45°C range. The rate is first order in bromate and in RhIII but zero order with respect to the substrate and to HClO4. The influence of Hg(OAc)2, ionic strength and Cl- on the rate was found to be insignificant. A transient complex, formed between RhIII and bromate ion in a slow and rate determining step, reacts with the polyhydric alcohol to give the product in a series of fast steps, which lead to regeneration of the catalyst. Activation parameters have been calculated and a suitable mechanism is proposed.