How Alkyl Halide Structure Affects E2 and SN2 Reaction Barriers: E2 Reactions Are as Sensitive as SN2 Reactions

High-level electronic structure calculations, including a continuum treatment of solvent, are employed to elucidate and quantify the effects of alkyl halide structure on the barriers of S(N)2 and E2 reactions. In cases where such comparisons are available, the results of these calculations show close agreement with solution experimental data. Structural factors investigated include alpha- and beta-methylation, adjacency to unsaturated functionality (allyl, benzyl, propargyl, alpha to carbonyl), ring size, and alpha-halogenation and cyanation. While the influence of these factors on S(N)2 reactivity is mostly well-known, the present study attempts to provide a broad comparison of both S(N)2 and E2 reactivity across many cases using a single methodology, so as to quantify relative reactivity trends. Despite the fact that most organic chemistry textbooks say far more about how structure affects S(N)2 reactions than about how it affects E2 reactions, the latter are just as sensitive to structural variation as are the former. This sensitivity of E2 reactions to structure is often underappreciated.