TY - JOUR
T1 - Production of carboxylic acids from aldehydes under hydrothermal conditions
T2 - A kinetics study of benzaldehyde
AU - Fecteau, Kristopher M.
AU - Gould, Ian
AU - Glein, Christopher R.
AU - Williams, Lynda
AU - Hartnett, Hilairy
AU - Shock, Everett
N1 - Funding Information:
Funding for this study was provided by the National Science Foundation (Grants OCE-0826588 and OCE-1357243). The authors thank Christine Roeger of the ASU glassblowing facility for production of the silica tubes. We also appreciate Brian Cherry and Steve Davidowski of the ASU Magnetic Resonance Research Center for assistance with the NMR study. This work benefited greatly from many discussions with other members of the hydrothermal organic chemistry (HOG) group at ASU, in particular Christiana Bockisch, Kristin Johnson, Kirtland Robinson, Jessie Shipp, and Ziming Yang.
Publisher Copyright:
© 2018 American Chemical Society.
PY - 2019/2/21
Y1 - 2019/2/21
N2 - 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 °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 (S ‡ ) 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.
AB - 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 °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 (S ‡ ) 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.
KW - Arrhenius
KW - Cannizzaro
KW - disproportionation
KW - experiment
KW - hydrothermal
KW - kinetics
KW - pH
KW - sedimentary basin
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U2 - 10.1021/acsearthspacechem.8b00130
DO - 10.1021/acsearthspacechem.8b00130
M3 - Article
AN - SCOPUS:85059677409
SN - 2472-3452
VL - 3
SP - 170
EP - 191
JO - ACS Earth and Space Chemistry
JF - ACS Earth and Space Chemistry
IS - 2
ER -