Theoretical Studies on the Hydride Ion Affinities of Carbocations

Carbocations are key intermediates in many organic reactions, and hence reaction mechanisms as well as reaction rates involving carbocation intermediates could be affected by their stabilities. Relative stabilities and factors influencing the stabilities of carbocations in solution have been well demonstrated, and one of important trends to stabilize them is to add additional alkyl groups. Nevertheless, the intrinsic stabilities of carbocations should be determined in the gas phase rather than in solution, because the relative stabilities of carbocations in solutions might be strongly influenced by the solvent effects due to their characteristic positive charge. For example, ionization of tert-butyl chloride, (CH3)3CCl → (CH3)3C++ Cl-, is highly endothermic by 153 kcal mol-1 in the gas phase indicating unobservably slow reaction. Whereas it has been well known that the ionization of tert-butyl chloride is feasible in solutions because of the solvation of the ions. Moreover, the difference in stabilities between tertiary and secondary alkyl cations is about 17 kcal mol-1 in the gas phase but only about 9.5 kcal mol-1 in the SO2ClF solution. However, unfortunately, quantitative data for the stabilities of carbocations in the gas phase are very limited, albeit some have been reported for simple carbocations.