The mechanism for nucleophilic substitution of α-carbonyl derivatives. Application of the valence-bond configuration mixing model

The valence-bond configuration mixing model (VBCM) is applied to the nucleophilic substitution reactions of α-carbonyl derivatives. The model appears to resolve satisfactorily a number of features of these reactions that current mechanisms have not dealt with. These include: (i) the dependence of the rate-enhancing efect of the carbonyl upon the nucleophilic strength of the entering group, (ii) the unusually large Hammett ρ value for the reaction of PhCOCH2Br with substituted pyridines, and (iii) the mechanism by which the rate-enhancing effect of the carbonyl group is transmitted to the reaction centre.