The substituent effects of methyl, cyclopropyl, and phenyl at the cationic center

The relative heats of protonation of a group of ketones with methyl, cyclopropyl, and phenyl substituents were measured in a variety of acid systems. The decreasing order of relative stabilities determined by the enthalpies was usually found to be c-Pr>Me>Ph. It was found that the relative order of stabilities due to the substituent is dependent upon the rest of the molecule. The results suggest that there is a cooperative effect between the substituents.

There exists a major discrepancy between conclusions based on enthalpies and predictions based on relative order of ion stabilities. The phenyl group always produces the least stable ion whereas 13 C chemical shifts indicate that phenyl is the best at delocalizing the positive charge.

Acidity function data were used to calculate equilibrium constants for protonation in high acid concentrations where protonation is essentially complete. The free-energies calculated from the equilibrium constants are combined with enthalpies of protonation measured in the same acid to yield the relative entropies of protonation. The entropy values indicate that the solvent is more ordered in the more exothermic reactions.