Abstract:
Decarboxylation of oxaloacetic acid is accelerated by hydrated metal ions. The thermodynamic data of activation AG*, AH* and AS* for the rate-determining process were determined. The main species involved in autodecarboxylation was identified as univalent hydrogen oxaloacetate anion. Plots of AH* against TAS* (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 AG* arising from an increase in TAS* much larger than the increase in AH*. The metal-ion accelerating effect in this particular case is essentially an entropy phenomenon.
