Alternative explanation for a saturation phenomenon exhibited by poly-4(5)-vinylimidazole and an anionic ester

Abstract

The hydrolysis of a negatively charged ester, sodium 3-nitro-4-acetoxybenzenesulfonate (NABS), catalyzed by poly-4(5)-vinylimidazole (PVIm) was reinvestigated under the conditions in which the substrate concentration was in excess of the catalyst concentration. The reaction was carried out under the same reaction conditions as previous work (28.5 vol-% ethanol–water, pH 7.1, Μ = 0.02, 26°C). Careful studies on the initial rate with a stopped-flow spectrophotometer revealed that the initial rate at high NABS concentration did not level off (saturate) as much as expected from the previous results. This difference was ascribed to two factors. One was that in the previous study, we treated up to 5% hydrolysis, but in the present work up to 1% in order to determine the initial rate. The other was the relatively slow deacylation reaction of the intermediate, partially acetylated PVIm, which resulted in less efficiency of the catalysis. The slow deacylation was confirmed by the kinetics of the hydrolysis of NABS and also independently by measuring the deacylation rate of the acetylated (or acylated) PVIm. The deacylation was an intramolecular catalysis and the rate constant was about 0.1 min —1 in 28.5 vol-% ethanol–water at pH 7.1 at 26°C. The whole process of the hydrolysis could be explained by a two-step pathway: an initial burst acylation followed by the slow deacylation. The electrostatic interaction between the cationically charged imidazole groups and NABS seemed not strong enough to form a stable complex. Thus, potassium p -toluenesulfonate showed no effect as a competing agent (inhibitor) against NABS.