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Keywords:

  • carbocations;
  • ionic liquids;
  • Lewis acids;
  • superacidic systems;
  • X-ray diffraction

Abstract

The room-temperature ionic liquid (RT-IL) [C(CH3)3]+ [Al2Br7] (m.p. 2 °C) was generated by bromide abstraction from tert-butyl bromide with the Lewis acid aluminum bromide in the absence of solvent. The crystal structure of the tert-butyl cation salt was determined by X-ray diffraction. NMR, IR, and Raman spectroscopy, as well as quantum-chemical and thermodynamic calculations, confirm the composition of this RT-IL. Thus, one may consider this RT-IL to be a readily accessible (and on a large scale) cationic Brønsted acid (protonated isobutene) with the potential for further reactivity. Based on the new absolute Brønsted acidity scale, we calculated an absolute pHabs value of 171 for liquid bulk [C(CH3)3]+ [Al2Br7]. This value is about as acidic as 100 % sulfuric acid (pHabs=171) and, thus, on the edge of superacidity.