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Rate constants (ks) of alkaline fading of crystal violet (CV+) have been determined at 25 °C by spectrophotometric measurements in aqueous mixtures of some protic, aprotic, and dipolar aprotic cosolvents. Transfer free energies of the substrate (CV+), [Formula: see text], were also determined in some of the solvent systems from solubility measurements of the chloride salt, and by subtracting [Formula: see text] obtained earlier by use of the tetraphenylarsonium tetraphenylboron (TATB) extrathermodynamic assumption. This helped determine transfer free energies of the transition state (X≠), [Formula: see text] values of lyate ion (S−) based on the TATB assumption are already known for all of these solvent systems. The observed log (ks/kw) – composition profiles reveal that the relative solvation of the reacting species rather than the dielectric constant of the solvents dictates the complex variation of the rates of the reaction in these solvent systems. Correlation of [Formula: see text] with [Formula: see text] indicates that the reaction is largely controlled by the relative solvation of S− in most of the cases. But analysis of [Formula: see text] – composition profiles for some of the solvent systems reveals that the non-compensation of the [Formula: see text] contributions of initial-state substrate and of the transition-state complex, which may be considered to be an outer-sphere complex [CV+](S−), is also in accord with what is expected from the relative solvating characteristics of the cosolvents as guided by their respective physico-chemical properties.

Deprotonation constants of phthalic acid ( H A ) , (KJHA, and biphthalic acid (HA-), (KJHA-, have been determined at 2 5 °C by measuring the e.m.f.s of galvanic cells comprising glass and Ag-AgCI electrodes in aqueous mixtures of organic cosolvents of different chemical nature, viz. protic glycerol (G L), aprotic dioxane (D), protophobic dipolar aprotic acetonitrile (ACN), and protophilic dipolar aprotic dimethyl sulphoxide (DMSO). Medium effects on deprotonation of the acids: 6(AG;i,) = 2.303 RT [p(&,)p(,K,)] have been dissected into transfer free energies AG: of the species involved by evaluating AG: of the uncharged acid ( H2A) from the measured solubilities of the acid and using AG: of H + based on the widely used tetraphenylarsonium tetraphenylboride (TATB) reference electrolyte assumption, as reported earlier for the solvents. The contributions of the different species involved in the protolytic equilibria, viz. H + , acids ( H A or HA-), and their respective conjugate bases (HA-or A2-) are discussed in terms of their solvation behaviour as guided by the 'acid-base', dispersion, structural, and electronic characteristics of the acid-base species and of the cosolvent molecules and their aqueous mixtures, besides the Born-type electrostatic interactions on the ionic acid-base species.Despite extensive studies 1-8 the medium effects on deprotonation or protolytic equilibria of weak Brlansted acids of different charge types can hardly be taken as completely understood. Previously these effects were believed to be chiefly guided by the change of dielectric constant of the solvents. But with the recent observation that the extent of some protolytic reactions is found to differ in different isodielectric solvent systems,8 it has been recognized that the dielectric constant cannot be the sole factor but the chemical nature of the cosolvents also plays an important role in dictating the overall solvent effects. But since the estimation of the dielectric constant or 'Born-type electrostatic effect', is still a difficult task, the 'chemical' effect is hard to discern. Consequently, the true understanding of the effect of changing solvents on protolytic equilibria remains an intriguing problem.Recently however, it has been increasingly recognized that another way of estimating the medium effect on protolytic equilibria of acids and bases is to dissect the contributions of different species involved in the reaction and to understand the behaviour of the individual species in the light of physicochemical properties like acidity, basicity, dispersion, structural, and electronic characteristics of the cosolvents. Evidently, one of the essential prerequisites for understanding the protolytic equilibria is the evolution of solvation energies or at least transfer free energies AG: , of the involved species from the reference solvent to the solvents concerned.Thus, as has been indicated earlier,8 understanding of the solvent effects on the deprotonation of acids (A") of different charge types becomes easier if we consider the ...

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]

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