The Inversion of Sucrose

Abstract

In a paper on electrolyte effects in reactions between a non-electrolyte and the hydrogen ion2 two typical examples were given, representative of electrolyte effects in (1) reactions showing specific catalysis by the hydrogen ion, and (2) reactions showing acid catalysis in the general sense. This classification has been clarified by experiments in solutions of heavy waterla and by Pedersen's analysis of prototropic reactions.'From an examination of the electrolyte effects it appeared that the inversion of sucrose and the hydrolysis of the acetals should be considered purely as catalysis by the hydrogen ion. Evidence of catalytic effects of acid molecules has been advanced by Hammett and Paull6 and by Gross, Steiner and Suess.' Experiments on the inversion of sucrose in acetateacetic acid buffer solutions have shown, however, there is no detectable catalytic effect of the molecules of such a weak acid as acetic?If we are dealing with a case of specific hydrogen-ion catalysis, we can for aqueous solutions postulate the equilibriumwhere kz << kand the equilibrium concentration is maintained at all times. The velocity constant is given by Now K will vary with the electrolyte concentration, and the resulting change in the concentration of the collision complex will cause a change in the measured velocity constant R. These changes have been observed for the hydrolysis of the acetals and for the inversion of sucrose, but even in dilute solution they seem larger than one would expect, by analogy, from an examination of the change, with electrolyte concentration, of the dis-(1) E. I. du Pont de Nemoun and Company Fellow, 1938. (2) Riesch and Kilpatrick, J . Phys. Chcm., 89, 561 (1935). (3) For references and discussion see Bonhoeffer, Trans. Faraday (4) Pedersen, ibid., 84, 237 (1938). (5) Hammett and Paul, THIS JOURNAL, 56, 830 (1934). (6) Gross, Steiner and Suess, Trans. Faraday Soc., 83, 883 (1936). (7) These experiments were camed out in this Laboratory in 1929 Sac., 84, 262 (1938). by M. L. Kilpatrick. sociation constant of anilinium or o-chloroaniliniumPedersen determined these two equilibrium constants over a convenient range of temperature, and found no great change in the heat of dissociation (MI') with temperature, in contrast to the marked variation of the heat of dissociation with temperature for uncharged acids. By analogy it might be expected that for reactions between a non-electrolyte and an ion, as represented by equation (1) , there would be little change with temperature in EA, the heat of activation computed from the integrated form of the Arrhenius equation (3)where k T , and kT, are the velocity constants at temperatures TI and Tz, respectively. This seems to be the case for the hydrolysis of diethyl acetals2An examination of the extensive literature on sucrose inversion (on which some excellent experimental work has been done) yields a n q b e r of surprises. For example, in a recent number of THIS JOURNAL we have "The activation energy of 25.7 f 0.05 kcal. found in the present work on sucrose is i...

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

“…3 In Table 1 are given the values of the velocity constant k(min-') (the number of moles of sucrose reacting per mole of sucrose per min) for the solvents at 25°C. Notably, the observed value of k in pure water compares fairly well with the value computed from Leininger and Kilpatrick's data (17) at different concentration of HC1.…”

“…As the kinetic behaviour of the reaction in all the aquoorganic solvents is found to be exactly similar to that in water, the reaction can be assumed to be a case of specific hydrogen ion catalysis as in water (17 For personal use only.…”

“…S(AG3) values were obtained after the Eyring equation Moreover, as the concentration dependence of S(AG3) is expected to be fairly small (vide ref. 17), S(AG3) can be tentatively related to transfer free energies of the species involved in the reaction as follows (7) The tentative values of the transfer free energies of the transition state, AG(x3) were then evaluated by use of the corresponding values of AG(S) and AG(H+) (10-12) for the solvents. All these values are presented in the table.…”

“…H+, S, and X* and also S(AG*) in the aqueous mixtures of the co-solvents GL and UH, and as compared to water. Evidently, the more pronounced destabilization observed of the transition state (X*) which is effectively a protonated sucrose [S.H30]+ (17), is attributable to the resultant effect of the steric hindrance of GL molecules and the decreased basicity of GL-water mixtures. Thus, it appears that although the transition state is increasingly destabilized relative to the initial state, the opposing resultant effects of increased destabilization of both H+ and initial-state substrate (S) overcompensate the effect of the former and thus enhance the rate of the reaction with increased proportion of GL.…”

Some Basics En Route to Isoconversional Methodology

“…As widely accepted [40], the birth of chemical kinetics is marked by the work of Wilhelmy, who in 1850 measured the rate of this reaction. It seems symptomatic that this initial reaction later became known [41] as having variable activation energy. The effect is illustrated in Fig.…”