Rate‐equilibrium LFER characterization of transition states: The interpretation of α

Abstract

AbstractThe effect of substituents on the rate of a reaction and the effect of the same substituents on the equilibrium can often be related by a linear free energy relation (LFER): log k− = α log K= + constant, where k+ and K= are the rate constant and the equilibrium constant, respectively. This review, concentrating on group transfers, adds to many studies describing the use of α to describe the transition state. Although the use of α to describe transition states is general, group transfers constitute a simple class allowing a fairly complete description yet illustrating two often neglected contributions. Group transfers can be described by the Marcus equation relating rate to an average identity rate and the equilibrium constant; a major contributor to the slope, α, of the rate‐equilibrium LFER is the variation of identity rates with substituent, rather than reflecting product‐like character. Substituent effect LFERs are predominantly attributable to interaction of charges with the substituent. However, α is not an exact measure of the charge on the substituent‐containing group, because in a transition state, but often not in a reaction product, there are more remote centers of charge which exert a smaller attenuated effect. A simple treatment of this attenuation for group transfers is proposed. The possibility of application of these ideas to proton transfer reactions and the interpretation of the Brønsted α (or β) is proposed.