Decarboxylative Ketonization Mechanism: Rates of Individual Steps
While any mechanism of a multistep reaction always remains a hypothesis, the strongest criteria for its testing comes from testing each individual step. Often, it is a difficult task, which may involve short lived intermediates and may not always be possible to achieve. The mechanism of the decarboxylative ketonization of carboxylic acids on metal oxides, important for biofuels upgrading and for the preparation of unsymmetrical ketones, has been proposed through adsorption and enolization of acids on surface, followed by condensation with an acid molecule adsorbed nearby, dehydration to a beta-keto carboxylate and its subsequent decarboxylation. The goal of the current study is to assess the rate of each individual step. The intrinsic rate of enolization of acetic and isobutyric acids on surface of monoclinic ZrO2 in the presence of D2O has been determined by measuring the rate of alpha protons exchange with deuterium using the McKay equation for reversible reactions approaching equilibrium. The rate of the condensation step has been estimated indirectly in reverse to the decomposition of beta-diketones by the retro-condensation reaction in the presence of water or methanol. Four beta keto acids have been prepared in a solid crystalline state, characterized by single crystal x-ray diffraction method, and deposited on surface of monoclinic ZrO2 to represent beta-keto carboxylate intermediates. The rate of their decarboxylation has been measured by monitoring pressure of the released CO2 which follows kinetics of a first order reaction. Thus, rates of all steps have been estimated and compared with the global rate of the decarboxylative ketonization reaction providing support for the proposed reaction mechanism. Reversibility of the catalytic mechanism has been confirmed experimentally and it will be discussed along with a considerable amount of previously unnoticed side reactions taking place.
Ignatchenko, Alexey; Springer, M.; and Ibrahim, R., "Decarboxylative Ketonization Mechanism: Rates of Individual Steps" (2019). Chemistry Faculty/Staff Publications. Paper 32.
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