Proton transfer from imidazole, benzimidazole, and their 1-alkyl derivatives. FMO analysis of the effect of methyl and benzo substitution

Article abstract of DOI:10.1139/v86-206

The rate-pH profile for detritiation from the C-2 position of 1-methylimidazole has been determined in aqueous solution at 85 deg C.The profile is consistent with a mechanism involving attack by hydroxide ion on the conjugate acid of the substrate to give an ylid intermediate in the rate-determining step.At higher pH, hydroxide-catalyzed exchange of the neutral species becomes increasingly important.Comparison of the second-order rate constants derived from the rate-pH profiles of imidazole, 1-methylimidazole, benzimidazole, and 1-methylbenzimidazole showed that methyl substitution caused the rate to increase by 2- to 3-fold while benzo annelation increased the rate by 10- to 20-fold.Frontier molecular orbital (FMO) analysis of the reaction scheme for proton transfer from imidazole, benzimidazole, and their 1-alkyl derivatives has been used to explain the rate-accelerating effect of methyl substitution and benzo annelation in these processes.