The pH Dependence of the Ruthenium-Catalyzed Ferricyanide Oxidation of Cyclohexanol

Abstract

The oxidation of cyclohexanol to cyclohexanone by ferricyanide in alkaline aqueous solutions is catalysed by the addition of chlororuthenium compounds. In solutions of pH less than 11 the progress of the reaction is limited by the decomposition of the catalyst in its reduced state. The catalyst lifetime can be lengthened by an increase in the concentration of the ferricyanide oxidant. In solutions of pH 11.3-11.9 either of the oxidation or the reduction steps of the catalytic cycle can be made rate determining, by adjustment of the relative concentrations of cyclohexanol and ferricyanide . The decrease in rate with increase in pH is due to the pH dependence of the reaction of the oxidized catalyst. The rate constant decreases from 26 to 15 dm³ mol^-1 s^-l between pH 11.3 and 11.9. The rate constant for the ferricyanide oxidation of the reduced catalyst is pH-independent at (6 ± 2)×10² dm³ mol^-1 s^-1 at 298 K, over the same pH range.