Markku Lahti

Kinetic acidity of carbon acids: initial and transition state enthalpies of transfer from water to aqueous dimethyl sulphoxide in the hydroxide ion catalysed 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.

Kinetic acidity of carbon acids: the hydroxide ion-catalysed ionization of chloroform and acetophenone in aqueous hexamethylphosphoric triamide

The rate coefficients for the detritiation of [3H]chloroform and [3H]acetophenone have been measured in aqueous hexamethylphosphoric triamide (HMPTA). The rate acceleration, caused by the HMPTA in the solvent is higher in the reaction of chloroform than in that of acetophenone. The difference is even greater than that observed earlier in a less basic solvent system, aqueous dimethyl sulfoxide (DMSO). The activation entropy for the detritiation of chloroform decreases sharply from the highly positive value in pure water to negative values at mole fractions of HMPTA higher than 0.1. The observed effects may be explained in terms of changes in solvation on going from the initial to the transition state. The change in the activation entropy in the reaction of chloroform is taken to indicate a decrease in the extent of internal return.