Electrochemical Aldehyde Oxidation at Gold Electrodes: gem-Diol, Non-Hydrated Aldehyde and Diolate as Electroactive Species.

To date the electroactive species of selective aldehyde oxidation to carboxylates at gold electrodes is usually assumed to be the diolate. It forms with high concentration only in very alkaline electrolytes, when OH- binds to the carbonyl carbon. Accordingly, the electrochemical upgrading of biomass derived aldehydes to carboxylates is believed to be limited to very alkaline electrolytes at many electrode materials. However, OH- induced aldehyde decomposition in these electrolytes prevents application of electrochemical aldehyde oxidation for the sustainable upgrading of biomass to value-added chemicals at industrial scale. Here, we demonstrate the successful oxidation of aliphatic aldehydes at a rotating gold electrode at pH 12, where only 1% of the aldehyde resides as the diolate species. This concentration is too small to account for the observed current, which shows that also other aldehyde species (i.e. the geminal diol and the non-hydrated aldehyde) are electroactive. This insight allows us to develop strategies to omit aldehyde decomposition, while achieving high current densities for the selective aldehyde oxidation, making its future industrial application viable.