A common carbanion intermediate in the recombination and proton-catalysed disproportionation of the carboxyl radical anion, CO2*-, in aqueous solution.

The carboxyl radical anion, CO2.−, was produced by the reactions of OH radicals with either CO or formic acid in aqueous solution. The pKa(.CO2H) was determined by pulse radiolysis with conductometric detection at pH≈2.3. The bimolecular decay rate constant of CO2.− (2 k≈1.4×109 dm3 mol−1 s−1) was found to be independent of pH in the range 3–8 at constant ionic strength. The yields of the products of the bimolecular decay of the carboxyl radicals, CO2 and the oxalate anion were found to depend strongly on the pH of the solution with an inflection point at pH 3.8. This pH dependence is explained by assuming a head-to-tail recombination of the CO2.− radicals followed by either rearrangement to oxalate or a protonation of the adduct, which subsequently leads to the formation of CO2 and formate. The recombination of CO2.− to give oxalate directly is estimated to have a contribution of <25 %.