Broensted superacidity of hydrochloric acid in a liquid chloroaluminate. Aluminum chloride - 1-ethyl-3-methyl-1H-imidazolium chloride (55.O m/o AlCl3)

Abstract

The system HCl (0.1-1 atm)/AlCl{sub 3}-EMIC (55.0 mol % AlCl{sub 3}) (EMIC = 1-ethyl-3-methyl-1H-imidazolium chloride) at 23{degree}C is a Broensted superacid capable of protonating arenes to a degree similar to that of liquid HF at 0{degree}C (H{sub 0} = {minus}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 < III and IV < V < VI < VII < VIII. A study of the effect of HCl partial pressure showed, for example that V is 50% protonated at 0.3 atm HCl. The overall reaction is arene + HCl + Al{sub 2}Cl{sub 7}{sup {minus}} {r reversible} arene {times} H{sup +} + 2AlCl{sub 4}{sup {minus}} 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 thatmore » yields exceptionally clean AlCl{sub 3}-EMIC melts with very low concentrations of protic and oxidizing impurities. 17 refs., 6 figs., 2 tabs.« less