\Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde

Aldehydes represent an intermediate redox state of organic carbon and can be precursors to carboxylic acids via disproportionation. A model aldehyde, benzaldehyde, was subjected to hydrothermal experiments (250-350 degrees C, saturation pressure) to assess the kinetics and mechanisms of the reactions leading to carboxylic acids. The concentration dependence demonstrates the kinetics are second-order in benzaldehyde, consistent with a disproportionation reaction, which is reminiscent of the base-promoted Cannizzaro reaction known at lower temperatures. Arrhenius parameters for these rate constants trend well with data from most, but not all, previous studies for the reaction under supercritical conditions. The rate constants yielded an entropy of activation (Delta S double dagger) of -161 J mol(-1) K-1, consistent with a bimolecular transition state at the rate-limiting step. Experimental yields of benzoic acid and benzyl alcohol were not equal, unlike what is expected for the disproportionation reaction. A kinetic model that included many secondary reactions was developed and was able to reproduce the observed yields of benzyl alcohol and benzoic acid, indicating that disproportionation and subsequent secondary reactions can explain the observed product yields. Experiments at different pH values demonstrated that while the reaction can be expedited by hydroxide, as traditionally understood, at lower pH the rate is independent of pH, indicating the neutral hydrate is capable of hydride donation to another benzaldehyde molecule in the rate-limiting step. Aldehyde disproportionation offers an alternative pathway to ketone oxidation for the formation of carboxylic acids in geologic environments where they are abundant, such as sedimentary basins.