Metal-ion catalyzed decarboxylation of oxaloacetic acid

Abstract

Decarboxylation of oxaloacetic acid is accelerated by hydrated metal ions. The thermodynamic data of activation ΔG^{[graphic omitted]}, ΔH^{[graphic omitted]} and ΔS^{[graphic omitted]} for the rate-determining process were determined. The main species involved in autodecarboxylation was identified as univalent hydrogen oxaloacetate anion. Plots of ΔH^{[graphic omitted]} against TΔS^{[graphic omitted]}(T= 298 K) for the decarboxylations in the presence of various hydrated divalent and tervalent metal ions and also in the absence of hydrated metal ion show excellent linearity.

A compensation effect was observed between the activation enthalpy and entropy terms. A barrier in the autodecarboxylation reaction is a highly entropy-decreasing process. The decarboxylation of a metal chelate complex, however, is facilitated by decrease in ΔG^{[graphic omitted]} arising from an increase in TΔS^{[graphic omitted]} much larger than the increase in ΔH^{[graphic omitted]}. The metal-ion accelerating effect in this particular case is essentially an entropy phenomenon.