Kinetics and mechanism of the oxidation of diols by acid permanganate

Abstract

The kinetics of the oxidation of four vicinal diols, four other diols, and two of their monoethers by acid permanganate have been studied. The vicinal diols yielded products arising out of glycol bond fission while other diols yielded hydroxycarbonyl compounds. The reaction is first order with respect to the diol and permanganate. The rate increases with an increase in the acidity. Permanganic acid has been postulated as the reactive oxidising species. The oxidation of [1,1,2,2-²H₄]ethane-1,2-diol showed the absence of a primary kinetic isotope effect. The values of solvent isotope effect, k(H₂O)/k(D₂O), at 298 K for the oxidation of ethane-1,2-diol, propane-1,3-diol, and 2-ethoxyethanol were 3.30, 0.43, and 0.40, respectively. Plots of (log k₁+H₀) against log a_H2O are linear in the case of vicinal diols with the slope, ω, having values from 3.45 to 3.92. This suggested proton abstraction by water in the rate-determining step. An acyclic mechanism involving glycol bond fission has been proposed for the vicinal diols. The other diols are oxidised by a hydride-transfer mechanism as are monohydric alcohols.