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Production of Carboxylic Acids from Aldehydes under Hydrothermal Conditions: A Kinetics Study of Benzaldehyde
journal contribution
posted on 2018-12-11, 00:00 authored by Kristopher M. Fecteau, Ian R. Gould, Christopher R. Glein, Lynda B. Williams, Hilairy E. Hartnett, Everett L. ShockAldehydes 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 °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 (ΔS‡) 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.