Calculations of steric effects. Part II. The SN2 halide exchange reactions

Abstract

Using the methods previously described, calculations have been carried out on steric effects in the S_N2 Br^––RBr halide exchange reactions. Three transition-state models were investigated, a stiff model, a flexible model with one bond-bending mode, and a plastic model with two bond-bending modes and one bond-stretching mode. With exactly the same set of covalent radii, van der Waals radii, and nonbonded potential functions that were used before, the increments δΔG[ΔG^‡(R)–ΔG^‡(Me)], δΔH^‡, and δΔS^‡ were well reproduced by the flexible and plastic models for R = Et, Prⁱ, Bu^t, Prⁿ, Buⁱ, and neopentyl. The most significant difference between results of the present calculations and those of Ingold and his co-workers in 1955 is that we are able to account for the observed values of the activation parameters purely in terms of steric effects, whereas in Ingold's calculations substantial polar effects of α-groups in the transition state were postulated to take place. It is shown that the necessity for the assumption of polar effects arises from the method of calculation of the nonbonded interactions.