KINETIC ACIDITY OF CARBON ACIDS - INITIAL AND TRANSITION-STATE ENTHALPIES OF TRANSFER FROM WATER TO AQUEOUS DIMETHYL-SULFOXIDE IN THE HYDROXIDE ION CATALYZED IONIZATION OF ACETOPHENONE

Enthalpies of solution of acetophenone in water, dimethyl sulphoxide, and mixtures of the two have been measured. These values, together with the enthalpies of activation and the enthalpies of transfer of hydroxide ion from water to the mixtures, have been used to calculate transition state enthalpies of transfer for the hydroxide ion catalysed ionization of acetophenone; these increase strongly with the dimethyl sulphoxide (DMSO) content of the solvent. The values are very close to those for the hydroxide ion. The enthalpically strong desolvation of the hydroxide ion on changing the solvent from water to aqueous DMSO is offset by the comparable desolvation of the transition state. For this reason the rate acceleration caused by the DMSO in the solvent is poor in the detritiation of [3H]acetophenone compared with that of [ 3H]chloroform.