Kinetics and mechanism of the osmium tetroxide catalysed oxidation of tartrate and malate ions with alkaline ferricyanide

Abstract

The alkaline ferricyanide oxidations of tartrate and malate ions have been studied in the presence of osmium tetroxide used as catalyst. The rates are consistent with a mechanism involving as the first step the formation of a 1 : 1 complex of osmium(v111) and the anion of the organic acid followed by decomposition to intermediate products and the osmium(v1) species with hydroxide ion. The osmium(V1) thus produced is rapidly oxidized to osmium(V111) with ferricyanide. The kinetic data obtained at higher tartrate concentrations indicate the formation of an osmium(v111)-(tartrate)₂ complex that is resistant to decomposition. The possibility of the formation of osmium(V1)-organic anion complexes, causing a retarding effect on the reaction velocity during a kinetic run, is also discussed. The oxidation products are determined and a possible set of reactions for their formation is set out.