Isoparametricity Phenomenon and Kinetic Enthalpy-Entropy Compensation Effect: Experimental Evidence Obtained by Investigating Pyridine-Catalyzed Reactions of Phenyloxirane with Benzoic Acids

摘要

The joint effect of structure and temperature on the rate and free activation energy of reactions between phenyloxirane and substituted benzoic acids catalyzed by substituted pyridines in acetonitrile has been investigated. A correlation analysis of the results of a multifactor kinetic experiment indicates the additivity of the joint effects of structural factors (substituents X in pyridines and substituents Y in benzoic acids) and of the effects of the substituents Y and temperature. Intensive interaction (nonadditivity) is observed between the effects of the substituents X and temperature. This fact has provided experimental evidence for the existence of the enthalpy-entropy compensation aspect of the isoparametricity phenomenon: at the iso-parametric temperature point (isokinetic temperature), the rate (free activation energy) of the process is independent of the structure of the substituent X because of the existence of an enthalpy-entropy compen-sation effect; on passing through this point, an inversion of the effect of X on the catalytic activity of pyridines takes place (isoparametricity paradox). At the isoparametric point with respect to the substituent X constant, there is no temperature effect on the reaction rate because of the activation enthalpy being close to zero. The isoparametric properties of a cross series of reactions are used to describe the mechanism of the pyridine-catalyzed opening of the oxirane ring.