Enthalpy-Entropy Compensation in Reactions of Oxirane Ring Opening

Results are summarized from a systematic study of the enthalpy-entropy compensation effect in non-catalytic and pyridine-catalyzed reactions of aryloxiranes with organic acids of various classes. This effect is observed in isoparametric (isokinetic, isoenergetic) reaction series, due to the interaction (nonadditivity) of the joint effects of temperature and structure. Experimental evidence of its physical reality in a number of cross reaction series is considered. In the context of the compensation effect, there are transitions from one state of reaction systems in which the enthalpy term of the free activation energy acquires a zero value ( increment H-not equal = 0, increment G(not equal) = -T Delta S-not equal) to another in which the contribution to the free energy of activation of the entropy term disappears (Delta S-not equal = 0, increment G(not equal) = Delta H-not equal). The character of activation processes when there is no enthalpy-entropy compensation is discussed.