Mechanistic change in the reactivity of substituted phenyl acetates over phenyl thiolacetates toward imidazole in aqueous phase

Abstract

The kinetics of aminolysis of several substituted phenyl acetates by imidazole is studied in aqueous medium at 20°C and an ionic strength of 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 272-401 nm), under excess free imidazole, pseudo-first-order rate constants (κ obs) are obtained. For the esters with good nucleofuges, the reaction follows clean second-order kinetics and the plots of (κ obs - κ H) against free imidazole concentration are linear at constant pH. The macroscopic nucleophilic substitution rate constants (κ N) are obtained as the slopes of these plots and found to be pH independent. For the esters with poor nucleofuges, a rate dependence on more than first power of the free imidazole and a linear dependence of κ′ 2 on free imidazole is observed. Accordingly, the microscopic rate constants for the assisted paths viz. κ ga and κ gb have been disseminated besides for simple bimolecular attack. The Brönsted-type plots and Hammett plots were constructed whose slope values are consistent with a stepwise mechanism through a bipolar tetrahedral addition intermediate whose formation or breakdown being rate determining for various paths. Comparison of this reaction of oxyesters with the earlier reported works on similar reaction of analogue thiolesters under identical reaction conditions showed remarkable mechanistic differences which are discussed in detail. The discussion is extended to include the details on previously studied ammonolysis of these two types of esters wherein thiolesters showed differed reactivity than that reported in the present study. © 2005 Wiley Periodicals, Inc.