Stabilities of Carbonium Ions in Solution. 12. Heats of Formation of Alkyl Chlorides as an Entree to Heats of Solvation of Aliphatic Carbonium Ions

Article abstract of DOI:10.1021/ja00530a004

Heats of formation (ΔH_f) of tertiary alkyl chlorides may be calculated from measurements of the heats of hydrochlorination of appropriate olefins at -50 deg C in CH2Cl2.Since many good values for heats of formation of the olefins have been published, the heats of hydrochlorination lead directly to ΔH_f for the chlorides, for which few previous values are available.The new data, when combined with previously reported heats of ionization for the chlorides, provide heats of formation for the carbonium ions.Relative values for ΔH_f for the carbonium ions are remarkably similar in the gas phase and in SO2ClF, thus supporting the proposal that differential solvation of carbonium ions is small in solvents such as SO2ClF.Because solvation energies are nearly constant for carbonium ions, their relative energies as calculated from quantum theory should apply directly to nonnucleophilic condensed phase values - a situation which is drastically different from that for ammonium or oxonium ions.We have shown previously the close relationship between ionization energies for alkyl chlorides in SO2ClF and their solvolysis rates in ethanol.The present results complete the series of data which are necessary for rigorously relating theoretical calculations of carbonium ion stability to solvolysis reaction rates in solution.Thereby, they help to explain the great success of the carbonium ion theory of organic chemistry.