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

摘要

The investigation of substituent effects is one of the most important tools for the determination of reaction mechanisms. While effects of substituents on equilibria and rates of chemical reactions are usually attributable to differences in enthalpy,changes in the entropy of activation, particularly for reactions of reactive intermediates (fast reactions), have been recognized to be the origin of observed substituent effects. For the reactions of para-substituted benzhydryl cations with xc-nucleo-philes, we now demonstrate that the change from enthalpic to entropic substituent effects occurs within these reaction series on going from slow to fast reactions. Consequently, the reactivity of the nucleophile determines whether changing substituents inthe electrophile affects DELTA H~(not =) or DELTA S~(not =).