Mechanistic studies of aquachlororhodium(III) catalysis in potassium iodate oxidation of 1,2-propanediol in acidic medium: A kinetic approach

The kinetics of homogeneously Rh(III) catalysed oxidation of 1, 2-propanediol by acidic solution of potassium iodate at constant ionic strength of the medium has been investigated at 308 K. The reaction follows complex kinetics, being first order in [KIO3], zero order in 1, 2-propanediol and inverse fractional order in both [H+] and [Cl-]. The first order rate constant increases linearly with increase in [Rh(III)] in its lower concentration range but as the concentration of Rh(III) is increased towards the higher range, the rate constant becomes almost constant, indicating positive fractional order in [Rh(III)]. Variation of ionic strength of the medium and successive addition of mercuric acetate (used as I ions scavenger) has a positive effect on the rate of the reaction. IO3- and [RhCl5(H2O)](-2) have been postulated as the reactive species of KIO3 and Rh(III), respectively. Activation parameters for the slow and rate determining step of the proposed mechanism, which involves the formation of the most activated complex, [Cl5Rh(HO -> Hg)](-), prior to the slowest step, have been obtained from rate measurements at 30, 35, 40 and 45 degrees C. The rate law conforming to the observed kinetic results has been derived as -d[KIO3]/dt = kK(1)K(2)[IO3-][Rh(III)](T)[Hg(II)](T)/[H+]([K-1 + Cl-]) + K1K2([Rh(III)](T) + [Hg(II)](T)). The main oxidation products have been identified as acetic acid and formic acid.