CARBOCATION FORMATION BY SELECTED HYDROCARBONS IN TRIMETHYLSULFONIUM BROMIDE-ALCL3/ALBR3-HBR AMBIENT-TEMPERATURE MOLTEN-SALTS

Using UV-visible and H-1-NMR spectroscopy, the protonation of several hydrocarbons was studied in the Bronsted superacid molten salts, AlCl3-TMSuBr-HBr and AlBr3-TMSuBr-HBr (TMSu(+) = trimethylsulfonium, AlX(3):TMSuBr mole ratio = 2) at ambient temperature. m-Xylene, mesitylene, pentamethylbenzene, hexamethylbenzene, and fluorene were completely protonated, and 1,1-diphenylethyl cation was formed in the AlCl3-based melt while the AlBr3-based melt allowed almost complete toluene protonation and ca. 30% benzene protonation (at ca. 0.02 M arene level) and diphenylmethyl cation generation. Since the NMR spectra of neutral and protonated arenes could be recorded for virtually identical solvents, it was possible to show that the positive charge distributions on the five sp(2) hybrid carbon atoms of pentamethylbenzenium ion obtained from the present H-1- and earlier C-13- NMR work were in agreement. The degradation of triphenylmethane in the two melts, with and without HBr, was shown to progress from the formation of triphenylmethyl cation, benzene, and diphenylmethane in less acid melts to the formation of triphenylmethyl cation, diphenylmethyl cation, benzene, and benzenium ion in AlBr3-TMSuBr-HBr.