Mechanism of beta keto acids decarboxylation in solutions is a long debated subject in literature. The effect of alkyl substituents on the rate of beta keto acids decarboxylation is not quite clear. Previous results of our DFT computational modeling of the concerted decarboxylation in the gas phase for four different molecules based on acetoacetic acid with a various degree of substitution indicate a noticeable electronic effect of alkyl substituents [1]. DFT results suggest a strong correlation of the donor-acceptor interactions in TS and reagents between carbonyl group and sigma bonding / anti bonding orbitals of the breaking bond with the electronic energy of activation. At the same time, overwhelming evidence has been accumulated in literature opposing the concerted mechanism.<br/>In the current work we have synthesized four alkyl substituted acetoacetic acids described above and measured rates of their decarboxylation in aqueous solutions as a function of temperature in the 30-55 °C range. In general, we have found lower activation energy for higher degree of alkyl substitution.<br/>Single crystal x-ray diffraction analysis shows that all four acids are crystallized by making hydrogen bonding in pairs. Carboxylic protons are turned away from the beta carbonyl group and cannot reach it. This explains stability of beta keto acids in crystalline state, since protonation of the beta carbonyl group is a condition required for the decarboxylation to occur.<br/>Experimentally derived Arrhenius activation energies in aqueous solutions in the decreasing order can be explained by the elongation of the breaking bond found in x-ray diffraction analysis in crystals. Previously calculated DFT electronic energies of the gas phase concerted model do not correlate with the experimental activation energies.<br/>All four beta keto acids are formed as intermediates in the proposed mechanism of the catalytic decarboxylative cross-ketonization of a mixture of acetic and isobutyric acids [1]. The rate of their decarboxylation on surface of zirconia catalysts at 250-450 °C temperature range computed by DFT does correlate with the experimental rate of decarboxylation in solutions extrapolated to the same temperatures, except for the most substituted beta keto acid derived from the isobutyric acid reaction with itself.

Effects of alkyl substituents on the kinetics of beta-keto acids decarboxylation in aqueous solutions

255th National Meeting (2018)

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