Effect of substituents on the geometry of transition states for slow proton transfer reactions

Abstract

Existing theories of substituent effects on the geometry of transition states in concerted reactions of the type: X + YZ → XY + Z are considered. The model of Thornton is revised, for cases in which Y is a proton, in that X–Z repulsive interactions are taken into account, and it is found that, under conditions where substitution which renders the basic centre of X more electron-rich is accompanied by an increase in the magnitude of Y–Z repulsions, the normal co-ordinates of the transition state may change so as to place the proton closer to the strengthened base. This effect contradicts the basic Thornton rules. The theoretical conclusion is examined in the light of the few available items of pertinent reactions (insofar as the proton transfer component in the latter are concerned), and cases which tend to support the theoretical conclusions are found.