New Mechanistic Interpretation of E2 Reactions

Abstract

The driving-force for the rate enhancement of α-substituents with a −M-effect in Sn2 and E2 reactions (U-shaped Hammett ρ-curves) has been re-examined. Instead of orbital overlap between the nucleophile and the −M-group (energetic acceleration), as most of the existing explanations state, the enhanced probability of the collision of the reaction partners (acceleration by entropy) is suggested to account for this phenomenon. A new transition state is proposed for the base-induced transeliminations which emphasizes the nucleophilic role of the base. The following problems involving elimination reactions will be discussed with the help of the new transition state: difference between thermodynamic and kinetic basicity; why trans-eliminations are generally favored over cis-eliminations; substituent isotope effects; orientation; competition between Sn2 and E2 reactions; and solvent effects in the debrominative decarboxylation reactions.