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

The protonation equilibria of methyl ketones RCOCH3 (R = Me, Et, i-Pr, c-C3H5, c-C6H11, t-Bu, Ph, p-MeOC6H4) symmetrical ketones RCOR (R = Et, i-Pr, c-C3H5, Ph), benzaldehyde, and methyl esters RCOOCH3 (R = Me, Et, i-Pr, c-C3H5, c-C6H11, t-Bu, Ph) have been investigated in aqueous sulfuric acid in the temperature range 25-90-degrees-C. The protonation parameters m* and pK(BH+) 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 (-delta-R-DELTA-G-degrees aq(BH+)) 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 -delta-R-DELTA-G-degrees aq(BH+). Enthalpies of hydration of BH+ are compared with values predicted by empirical correlations.