Variable Trends in R−X Bond Dissociation Energies (R = Me, Et, i-Pr, t-Bu)

High level ab initio molecular orbital calculations confirm experimental indications that the effect of alkyl substituents (R = Me, Et, i-Pr, t-Bu) on R−X bond dissociation energies varies considerably according to the nature of X. A simple qualitative explanation in terms of valence-bond theory is presented, highlighting the increasing importance of the stabilization of R−X by the ionic R+X- configuration for electronegative X substituents (such as F, OH, and OCH3).