ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 33, Number 1
JKCSEZ 33(1)
February 20, 1989 

This study deals with micellar effects on hydrolyses of p-nitrophenyl carboxylic esters(p-NPCE) mediated by 2-alkylbenzimidazole(R-BI) and sodium 2-alkylbenzimidazole-5-sulfonate(R-BISO3Na) in aqueous and CTABr solutions. The reactions mediated by R-BI and R-BISO3Na in micellar solutions are obviously slower than those by benzimidazole(BI) and sodium benzimidazole-5-sulfonate(BISO3Na) respectively, and the reaction rates were decreased with increase of lengths of alkyl groups. This prestents a striking contrast to the reactions in aqueous solutions without added CTABr, of which the reaction rates are on approximately same levels. It seems due to steric effect of alkyl groups for R-BI and R-BISO3Na in the Stern layer of micelle, and it is supported by measured activation parameters(ΔH≠, ΔG≠ and ΔS≠) of the reactions in aqueous and micellar solutions. In addition to nucleophilic ability of benzimidazole(BI) moiety of R-BISO3Na on the reactions, these compounds with long alkyl groups(nonyl to pentadecyl) which form a micelle of themselves increase the reaction rates due to their micellar catalyses in aqueous solutions, not including CTABr. We measured the isotope effects to elucidate the mechanism of hydrolyses of p-nitrophenyl carboxylic esters, and the relative first order rate constant(k'H2O/k'D2O) are on range of 2.5∼3.2. This range is too high to conclude that the hydrolyses of p-NPA mediated by various R-BISO3Na proceed by nucleophilic mechanism. In other words, the reactions are assumed to proceed in part by general basic one, as compared with the reaction catalyzed by imidazole(IM) in aqueous solution.