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Enthalpies of solution (AHS) of a variety of straight-chain, branched, and cyclic alkanes and alkenes have been measured in methanol, TV.iV-dimethylformamide (DMF), benzene, and cyclohexane. AHS values and enthalpies of evaporation {AHV) have been combined to give enthalpies of transfer from the vapor to each solvent [ //( -S)]. These solvation enthalpies are more exothermic for alkenes than for the corresponding alkanes in transfers to methanol, DMF, and benzene, by about 0.3, 0.6, and 0.3 kcal/mol, respectively, but are similar in transfers to cyclohexane. This suggests that solute-solvent interactions in cyclohexane are mainly dispersion forces. The more exothermic interaction of DMF with alkenes relative to alkanes results mainly from dipole-induced dipole interactions of the solvent dipoles with the highly polarizable bonds. Only very large steric factors inhibit the approach of solvent dipoles to the tr bond, as in trans-1,2-di-terr-butylethylene, where the alkene-alkane difference in AH(v -* S) is diminished. AH(v -* S) of alkanes and alkenes are reduced by nearly l kcal/mol for each quaternary carbon atom present, possibly arising from steric hindrance to solvation by dispersion forces.

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Enthalpies of solution of ethyl acetate in water, DMSO, and nine aqueous DMSO mixtures, and of tetraphenylphosphonium bromide, sodium tetraphenylborate, sodium bromide, and sodium hydroxide in two aqueous DMSO mixtures have been measured. Using the extrathermodynamic assumption AAHJPhiP -) = 8( 1 4 ~) enthalpies of transfer of hydroxide ion from water to aqueous DMSO mixtures have been calculated. AAHS values for ethyl acetate have also been obtained. Transition state enthalpies of transfer for the alkaline hydrolysis reaction have been derived from the 8 values and experimental enthalpies of activation. Above 15 mol % DMSO 8 (transition state) is more positive than AAHS (reactants). Thus, the increasing reaction rates observed with increasing DMSO concentration do not result from the large enthalpy of desolvation of hydroxide ion which is compensated by desolvation of the transition state, but, rather, by an entropy effect.

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