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Hydration of ketones in mixtures of water and dipolar aprotic solvents Van Dyke, John D.

Abstract

The hydration of a series of α,α,α-trifluoroacetophenones has been studied in sulfolane-water and dimethyl sulfoxide-water solvent systems and their extents of hydration measured by ¹⁹F N.M.R. spectroscopy and U.V. spectroscopy. In order to quantitatively determine the equilibrium constants for the very readily hydrated ketones in this series and at the same time to study the effect of a dipolar aprotic solvent on the ability of an aqueous medium to hydrate a ketone, a function W₀ (analogous to the Hammett acidity function H₀) was developed for the systems sulfolane-water and dimethyl sulfoxide-water. Nine substituted α,α,α -trifluoroacetophenones were used to establish the W₀ scale for sulfolane-water; two were used to establish the scale for dimethyl sulfoxide-water. The ability of a medium to reduce the extent of ketone hydration is indicated by an increase in the medium's W₀ function. In sulfolane-water, W₀ values increase continuously as sulfolane is added to the medium, ranging from 0 in pure water to about 2.7 in 99 mole % sulfolane. The corresponding function in dimethyl sulfoxide-water decreases initially as dimethyl sulfoxide (DMSO) is added reaching a minimum of -0.42 at 40 mole % DMSO. As more DMSO is added, the function rises and reaches a value of about 1.1 in 99 mole % DMSO. The differences between the W₀ functions in DMSO and sulfolane are accounted for by differences in the behavior of the water activity terms and the activity coefficients of the ketone hydrates in the two solvent systems. From considerations of the effect of the dipolar aprotic solvents on the hydrate molecules, it appears that the activity coefficients of the hydrates are affected in the same manner as the activity coefficients of water but to a greater degree. Data on the hydration of s-dichloroacetone in dioxane-water and acetonitrile-water (supplied by Professor R.P. Bell) allowed the development of functions in these solvents. A comparison of W₀ functions shows the following order for effectiveness of a solvent in decreasing the extent of hydration: sulfolane > acetonitrile > dioxane > DMSO. The pK[subscript d] (d = dehydration) values for the nine substituted α,α,α -trifluoroacetophenones determined in sulfolane-water vary from -0.86 for the 4-dimethylamino derivative to 3.15 for the 3-nitro derivative. A plot of log K[subscript d] versus the Hammett substituent constants (ϭ⁺) for these compounds yields a ρ⁺value of -1.62. Measurements of the kinetics of hydration in sulfolane-water mixtures containing no acid indicate that three water molecules are intimately associated with the hydrating ketone in the transition state for the water-catalyzed mechanism. In sulfolane-water mixtures containing acid, the same measurements do not indicate conclusively the number of water molecules associated with the transition state. The heats of reaction (ΔH) and entropies of reaction (ΔS) for the dehydration of α,α,α -trifluoroacetophenone hydrate and its 4-methoxy derivative were determined in sulfolane-water mixtures. ΔH values range between 7 and 10 kcal/mole and ΔS values between 19 and 25 e.u. Changes in solvent composition and ketone structure appear to have relatively little effect on ΔH and ΔS.

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