Understanding E2 versus S_N2 competition under acidic and basic conditions

Our purpose is to understand the mechanism through which pH affects the competition between base-induced elimination and substitution. To this end, we have quantum chemically investigated the competition between elimination and substitution pathways in H₂O+C₂H₅OH ₂⁺ and OH^-+C₂H₅OH, that is, two related model systems that represent, in a generic manner, the same reaction under acidic and basic conditions, respectively. We find that substitution is favored in the acidic case while elimination prevails under basic conditions. Activation-strain analyses of the reaction profiles reveal that the switch in preferred reactivity from substitution to elimination, if one goes from acidic to basic catalysis, is related to (1) the higher basicity of the deprotonated base, and (2) the change in character of the substrates LUMO from C^β-H bonding in C₂H₅OH ₂⁺ to C^β-H antibonding in C ₂H₅OH. Titrating the LUMO: The character of the LUMO of a substrate CH₃CH₂-LG (LG=leaving group) is found to be pH dependent: whereas it is C^β-H bonding under acidic conditions, it is C^β-H antibonding under basic conditions. This is an important factor behind the corresponding shift in reactivity towards an electron-donating agent Nu from substitution to elimination.