The valence-bond configuration mixing (VBCM) model of reactivity is applied to the problem of rate-equilibrium relationships. Reactions are divided into two classes: (a) the special case of two-configuration reactions, such as the SN2 reaction of methyl derivatives, which can be adequately described by just reactant and product configurations, and (b) multiple-configuration reactions, which incorporate most organic reactions, are exemplified by the SN2 reaction of benzyl derivatives, and require three or more configurations for an adequate description of the reaction profile. It is demonstrated with simple qualitative arguments that two-configuration reactions are likelyto show normal rate-equilibrium behavior if the substituents are adjacent to a site within the molecule at which charge is either generated or destroyed in the product. Insuch cases the product configuration is stabilized with respect to the reactant configuration in accord with the Bell-Evans-Polanyimodel. Anomalous (i.e., α > 1) or no rate-equilibrium relationships are anticipated if there is no change in charge adjacent to the substituent site or large steric interactions are generated in the transition state (TS). For multiple-configuration reactions, anomalous rate-equilibrium relationships are likely to result if the substituent change operates on just an “intermediate” configuration but normal rate-equilibrium relationships are expected if the substituent effect operates on the product configuration. The analysis suggests that the Brønsted parameter α (or β) fails to provide a measure of TS structure or charge development for two-configuration reactions but may well provide a relative measure of TS chargedevelopment for multiple-configuration reactions. The most striking conclusion is that anomalous α values are really quite common and depend primarily on the position of substitution within a reaction family, rather than on the reaction type.
ASJC Scopus subject areas
- Organic Chemistry