Thermochemical studies of carbonyl reactions. 4. Enthalpies of hydrolysis of norbornyl ketals

The dimethyl ketals of norbornanone and of eight methyl-substituted norbornanones were prepared, and the enthalpies of hydrolysis were determined. The compounds were chosen to provide a variety of steric interactions between the methyl substituents and the ketal group, much of which would be relieved on going to the ketone. The enthalpies of reaction varied by over 4 kcal/mol. The experimental data were modeled by molecular mechanics (MM2), and although a good correlation was found for the less substituted compounds, the ketal of camphor fell off the slope = 1 correlation line. The free energy changes were determined, and were found not to be well correlated with the enthalpy changes.