Broensted Superacidity of HCl in a Liquid Chloroaluminate. AlCl3-1-Ethyl-3-methyl-1H-imidazolium Chloride

Article abstract of DOI:10.1021/ja00184a020

The system HCl (0.1 - 1 atm)/AlCl3-EMIC (55.0 mol percent AlCl3)(EMIC = 1-ethyl-3-methyl-1H-imidazolium chloride) at 23 deg C is a Broensted superacid capable of protonating arenes to a degree similar to that of liquid HF at 0 deg C (H₀=-15.1).Arenes used in this investigation were biphenyl (I), naphthalene (II), 9H-fluorene (III), chrysene (IV), 2-methylnaphthalene (V), mesitylene (VI), pentamethylbenzene (VII), hexamethylbenzene (VIII), anthracene (IX), and 9,10-dimethylanthracene (X).In both the chloroaluminate melt and HF I is a weak base while VIII-X are strong bases.In between these extremes the order of basicities in both media is II arene*H(1+) + 2AlCl4(1-) and is reversible.The degree of protonation was measured by optical absorption spectrophotometry.The arenes are stable in the liquid chloroaluminate for many hours, and their protonated forms (arenium ions) are stable for 1 h or more.A new procedure for the preparation of EMIC was developed that yields exceptionally clean AlCl3-EMIC melts with very low concentrations of protic and oxidizing impurities.