Common Origin of Enthalpic and Entropic Substituent Effects in Reactions of Benzhydryl Cations with Nucleophiles

As long as an enthalpic barrier is present, a decrease in the electron-releasing ability of X and Y causes an increase in the reaction rates of reaction (a) due to the lowering of the activation enthalpy, while the activation entropy remains constant. When the enthalpic barrier disappears, further decrease in the electron-releasing ability of X and Y is associated with an increase in reactivity caused by an increase in the activation entropy.