Ionic solvation in aqueous dimethyl sulfoxide

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1972

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Abstract

Enthalpies of solution of tetraphenylphosphonium bromide, sodium tetraphenylborate, sodium bromide, sodium iodide, sodium hydroxide, ethyl acetate, and tetra-n-butylammonium chloride, bromide, and iodide have been measured at 25[degrees] in water, DMSO, and seven water-DMSO mixtures. The enthalpy of solution of tetra-n-butyltin in DMSO has been measured. Enthalpies of solution of n-propyl tosylate in mixtures containing greater than 50% by volume DMSO were measured. Using the enthalpy of transfer assumption [delta][delta]H[(C[lowered 6]H[lowered 5])[lowered 4]P[raised +]] = [delta][delta]H[[raised -]B(C[lowered 6]H[lowered 5])[lowered 4]P[raised +]], [delta][delta]H values have been calculated for (n-C^Hg)[lowered 4]N[raised +], Na[raised +], [raised -]OH, Cl[raised -], Br[raised -], and I[raised -] ions. The significant trends in single ion AAH values are (1) the tetra-n-butylammonium ion is desolvated by the addition of DMSO to water, the maximum desolvation being reached when the concentration of DMSO is about 65% by volume; (2) the sodium ion is initially slightly desolvated by the addition of DMSO to water, but the ion gains in solvation when the DMSO concentration begins to exceed about 30% by volume; (3) all of the anions are slightly more solvated by the addition of a small amount of DMSO to water. When the DMSO concentration exceeds about 30% by volume the anions are desolvated, the desolvation for hydroxide ion being quite pronounced. The halide ions reach a maximum desolvation when the DMSO concentration is about 80% by volume. The [delta][delta]H values for nydroxide ion and ethyl acetate have been used in conjunction with kinetic data from the literature to calculate AAH values for the transition state of the base-catalyzed hydrolysis of ethyl acetate. The relative contributions of changes in anion, substrate, and transition state solvation upon addition of DMSO to water, to the observed changes in the rate of base-chtalyzed hydrolysis of ethyl acetate are discussed.

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