Mechanism of the dissociative electro-oxidation of oxalate in aprotic solvents

Abstract

The oxidation of oxalate has been studied in dipolar aprotic solvents. The process is characterized by two voltammetric oxidation peaks and proceeds by a dissociative electron transfer mechanism, in which two carbon dioxide molecules are formed with consumption of 2 F mol-1. Voltammetric and convolution analyses of the first peak are in agreement with a stepwise mechanism in which a radical anion forms and then undergoes C-C bond cleavage. By analysis of the potential dependence of the transfer coefficient α, the standard potential was estimated to be 0.06 V vs. SCE in DMF. The carbon dioxide molecules released at the first peak interact with the starting material itself to form an adduct that can be oxidized at the second peak. As a consequence, when the potential is positive enough both the free and the complexed oxalate ions compete in the overall electro-oxidation, as supported by convolution analysis of the heterogeneous electron transfer at the second peak. The voltammetric pattern is quite sensitive to the presence of acids. In the presence of water, in particular, considerable positive shifts of the actual potential for the oxidation of oxalate were observed. This has been related to the formation of a water-oxalate adduct, although the actual species undergoing the electro-oxidation seems to be always the free oxalate ion. The consequences of this effect are discussed.