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

The kinetics of the Rh-III 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 degrees C range. The rate is first order in bromate and in Rh-III 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 Rh-III 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.