Nucleophilicities of Alcohols and Water in Acetonitrile Based on Reactivities of Benzhydrylium Ions

Abstract

Abstract The decay rate constants of benzhydrylium ions generated by laser irradiation in acetonitrile have been determined photometrically. The decay of the stable carbocation with an electron-releasing substituent was decelerated with addition of LiClO4, being attributed to the decrease of the recombination of the cations with the counter anions. On the contrary, the decay of the highly reactive carbocation was accelerated, indicating the occurrence of the reaction with perchlorate anion. The second-order rate constants (k2) for the reactions of benzhydrylium ions with methanol, ethanol, i-propanol, t-butanol, and water were determined in acetonitrile in the presence of 3 mM LiClO4 for benzhydrylium ions less electrophilic than 3,5-dimethyl-4′-chlorobenzhydrylium ion and in the absence of LiClO4 for the remaining reactive benzhydrylium ions. Their logarithms were linearly correlated with the electrophilicity parameters (E) in Mayr’s equation, log k2 = sN(E + N). Based on the correlations the nucleophilicity parameters (N) were evaluated for these nucleophiles in acetonitrile, MeOH, 6.86; EtOH, 7.13; i-PrOH, 6.82; t-BuOH, 5.35; H2O, 5.79. The low nucleophilicity of t-butanol regardless of its high gas-phase basicity representing the inherent nature in molecular structure suggests the importance of steric environment at the transition state of the combination of benzhydrylium ions with nucleophiles.