Stabilities of Carbocations in Solution. 14. An Extended Thermochemical Scale of Carbocation Stabilities in a Common Superacid

Article abstract of DOI:10.1021/ja00347a060

Until now, thermodynamic stabilities of carbocations have been limited to (1) relatively stable resonance delocalized ions which are ranked on the pK_R scale and (2) relatively unstable aliphatic and alicyclic ions which have been compared in the gas phase by ion cyclotron resonance or by calorimetry in SbF5/SO2ClF superacid at -50 to -120 deg C in our laboratory.The present paper will present an extensive series of new measurements which is designed to close the gap between the stable triarylmethyl cations and the unstable ions so as to put them all on a common energy scale.Carbinols were used as precursors in SbF5/HSO3F/SO2ClF at -40 deg C.As we reported recently (J.Am.Chem.Soc., 104, 3522(1982)), these conditions are necessary to avoid complications which appear to be introduced by ion-pairing when the alcohols are treated with the SbF5/SO2ClF system which was used previously to ionize alkyl chlorides.The results from the present work place 39 typical carbocations representing saturated, secondary, and tertiary, and aliphatic, bicyclic, and substituted cumyl, benzhydryl, and trityl systems on a common scale.Correlations and interpolation equations for relating other measurements in the gas phase and solution will be presented.The results provide useful comparisons of the ?⁺ and ?^C+ scales for correlating carbocation stabilities and provide new data for several classic questions in the field such as the ranking of methyl, phenyl, and cyclopropyl groups for stabilizing ions and also the reactions of carbocations with water.