THERMODYNAMICS OF PROTONATION OF KETONES AND ESTERS AND ENERGIES OF HYDRATION THEIR CONJUGATE ACIDS

The protonation equilibria of methyl ketones RCOCH3, (R = Me, Et, i-Pr, c-C3H5, c-C6H12, t-Bu, Ph, pMeOC6H4), symmetrical ketones RCOR (R = Et, i-Pr, c-C3H5, Ph), benzaldehyde, and methyl esters RCOOCH3 (R = Me, Et, i-Pr, c-C3H5, c-C6H12, t-Bu, Ph) have been investigated in aqueous sulfuric acid in the temperature range 25-90 C. The protonation parameters m* and pKBH+ have been derived, along with enthalpies and entropies of ionization. Structural effects on basicity are discussed in terms of the competition between internal and external stabilization by solvation. Solution- and gas-phase data have been combined to obtain the solvent effect on free energies and enthalpies of ionization and differential free energies and enthalpies of hydration of the protonated bases. The values of m* for compounds with a similar substitution pattern around the basic site are correlated with free energies. Enthalpies of hydration of BH+ are compared with values predicted by empirical correlations.