Transition-State Expansion: A Quantitative Model for Counterion Effects in Ionic Reactions

Ionic reactions are the most common reactions used in chemical synthesis. In relatively low dielectric constant solvents (e.g., dichloromethane, toluene), ions usually exist as ion pairs. Despite the importance of counterions, a quantitative description of how the paired 'counterion' affects the reaction kinetic is still elusive. We introduce a general and quantitative model, namely transition-state expansion (TSE), that describes how the size of a counterion affects the transition-state structure and the kinetics of an ionic reaction. This model could rationalize the counterion effects in nucleophilic substitutions and gold-catalyzed enyne cycloisomerizations.