Molecular basis of linear free energy relationships. The nature of inductive effect in aliphatic series

The nature of inductive substituent effect on the acidity of aliphatic carboxylic acids was analyzed in terms of a quantum chemical model based on the Longuet-Higgins theory of proton affinity. It has been shown that the effect is generally composed of two additive components, the electrostatic one and the polarization one. The relative weights of these components depend in any given case on the type of the reaction series. Thus, for example, the substituent-induced variation of gas-phase acidity in a series of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids is due to the electrostatic component classically described by the Kirkwood-Westheimer theory. On the other hand, the gas-phase basicity in a series of aliphatic amines is primarily determined by the polarization term that in traditional description corresponds to the concept of inductive effect.