Reactivity-Selectivity Relationships. A Quantum Mechanical Approach to Transition State Structure. Application to the SN2 Reaction of Benzyl Derivatives

A model for estimating the effect of substituents on transition-state structure in SN2 reactions is presented. The model is based on a description of the reaction complex in terms of a wave function built up from a linear combination of reactant configurations. Stabilization of a given configuration by a substituent effect is indicated to enhance its contribution to the wave function and hence to the energy and structure of the transition state. The structural effects of the substituent perturbation on the transition state are assessed by analysis of the structural requirements inherent within each of the configurations. The model is applied to the SN2 reaction of benzyl derivatives, and the predictions are compared with experiments and the earlier models. Within a limited family, substituent effects are likely to affect the transition state in an “anti-Hammond” manner. Only for comparisons of substrates of widely differing structure and reactivity are “Hammond” effects likely to manifest themselves in addition to the “anti-Hammond” behavior.