| Our laboratory has developed a theory of solvent effects in which the total solvent effect is partitioned into three contributing components: (1) the general medium effect (which arises from solvent-solvent interactions); (2) the intersolute effect (the consequence of solute-solute interactions); and (3) the solvation effect (which is the consequence of solute-solute interactions). We have applied our theory to describe solvent effects on the equilibrium processes of solubility, surface tension, and molecular complex formation in binary-aqueous organic mixtures. The objective of the present work is to extend our theoretical framework to describe solvent effects on chemical reaction rates.; The first of two systems that were studied is the decomposition of aspartame (aspartylphenylalanine methyl ester). At 25{dollar}spcirc{dollar}C, aspartame can degrade by hydrolysis to form aspartylphenylalanine, or by intramolecular aminolysis to form a diketopiperazine. The second system chosen for study was the hydrolysis of L-phenylalanine methyl ester, which, in some respects, can serve as a model for aspartame. A reversed-phase high-performance liquid chromatography assay for the quantitative analysis of aspartame, phenylalanine methyl ester, and their degradates was systematically developed. The development and final assay are presented. A pH-rate profile for both esters in buffered aqueous solutions was determined over a wide pH range (0.28-11.60).; Solvent studies were performed for five reactions: the acid-catalyzed hydrolysis of aspartame and phenylalanine methyl ester; the base-promoted hydrolysis of aspartame and phenylalanine methyl ester; and the base-promoted aminolysis of aspartame. The observed solvent effects were successfully modelled using our theory. The curve-fit parameters that were obtained were, for the most part, chemically reasonable with occasional anomalies. |
