Characterization of ester hydrolysis in terms of microscopic rate constants

Abstract

Hydroxide- catalyzed ester hydrolysis for molecules of coexisting species is quantitated in terms of microscopic rate constants, a new, species- specific physicochemical parameter. Relationships between the overall and component reactions, as well as the macroscopic and microscopic rate constants are deduced. Experimental techniques, evaluation methods, and feasibility are discussed. Species- specific, pH- independent rate constants of four coexisting, differently hydrolyzing microspecies are determined for the first time. Protonation of an alpha- amino and beta- imidazolyl site in amino acid esters has been found to accelerate the hydroxide- catalyzed hydrolysis by factors of 120 and 7.5, respectively, whereas they jointly exert a nearly 3000- fold acceleration. A total of 20 microscopic protonation equilibrium constants, as component parameters in the rate equations, have also been determined. The species- specific rate constants have been found to correlate with the site- and species- specific basicity of the leaving group and the NMR chemical shift of an adjacent proton. Individual contributions of the various microforms to the overall hydrolysis rate are depicted in microscopic reaction fraction diagrams.