Article

Abstract

The kinetics of methanolysis of acetyl imidazole (1) and acetyl pyrazole (2) have been investigated under anhydrous conditions in the presence of Zn(ClO4)2, Co(ClO4)2, and HClO4 at 25°C. In all cases, the plots of the pseudo-first-order rate constant for methanolysis (kobs) vs. [metal ion] or [HClO4] show saturation behavior indicative of equilibrium binding of the M2+ or H+ to the amide. Relative to the spontaneous methanolysis rate constant (ko), the catalytic rate constant obtained at saturation, kcat, is larger for metal-ion catalysis than for H+ catalysis. The (kcatH+/ko) ratio is 10.7 and 1.25 for 1 and 2, respectively, while the (kcatM2+/ko) for these divalent metals varies from 150-fold for 1 to between 700 and 5700-fold for 2. By contrast, in water, proton is far more effective at promoting the hydrolysis of 1 than are metals, the aqueous (kcatH+/ko) ratio being 560, while the (kcatZn2+ /ko) and (kcatNi2+/ko) ratios are 15 and 3.2, respectively.Key words: methanolysis, kinetics, metal-ion catalysis, acetyl imidazole, acetyl pyrazole.