Kinetic solvent effects on acid-catalyzed hydrolysis of sucrose in aqueous mixtures of some protic, aprotic, and dipolar aprotic solvents

Abstract

URMILA MANDAL, KAUSHIK DAS, and KIRON KUMAR KUNDU. Can. J. Chem. 64, 1638 (1986). Rate constants of acid-catalyzed hydrolysis of sucrose (S) to D-glucose and L-fructose have been determined at 25OC by optical rotation measurements in aqueous mixtures of protophobic protic glycerol (GL), protophilic protic urea (UH), aprotic dioxane (D), and dipolar aprotic dimethyl sulphoxide (DMSO). Transfer free energies of the substrate sucrose, AG(S) have also been determined in the solvents from solubility measurements. These values as well as those of H+, as obtained earlier by use of the widely used tetraphenylarsonium tetraphenylboron (TATB) reference electrolyte assumption, yielded transfer free energies of the transition state. The observed log (ks/kw) -composition profiles reveal that the rates increase monotonically in GL-water mixtures, that decrease more or less monotonically in UH-and DMSO-water mixtures, and decrease up to 10 mol% D in D-water mixtures, beyond which the values tend to increase. Examination of hG(i)-composition profiles for the different species in each case indicates that the initial and transition state solvation get more or less compensated and the observed rates are dictated by the increased solvation of H+ in aqueous UH, DMSO, and D co-solvent systems. But in GL-water mixtures the decreased solvation of the transition state compared with the initial state is overcome by the decreased solvation of H+, thus resulting in the gradual enhancement of the rates of the reaction. The observed linearity of the correlative plots of -6(AG*) [=RT In (ks/kw)] vs. A@,(H+) with distinctly different slopes in the two cases also substantiates the relative importance of H+ solvation in dictating the rates of the reaction in these widely different aqueous co-solvents. sont lintaires et qu'elles possCdent des pentes ICgkrement differentes; ces domkes confirment la relative importance de la solvatation des ions H+ dans I'orientation de la reaction dans ces diffkrentes co-solvants aqueux.[Traduit par la revue]

Kinetics of Formation of Di-d-fructose Dianhydrides during Thermal Treatment of Inulin

“…Aqueous acidified sucrose has been shown to undergo specific acid catalyzed hydrolysis. Mandal et al (1986) found that the kinetic behavior of acid-catalyzed sucrose hydrolysis in four nonprotic solvents was similar to that in water and therefore assumed specific acid catalysis. Lo ¨nnberg and Gyle ´n (1983) found the rate-limiting step in hydrolysis of alkyl fructofuranosides to be the formation of the fructosyl carbocation and not the protonation of the glycosidic oxygen.…”