Superacidic or Not…? Synthesis, Characterisation, and Acidity of the Room-Temperature Ionic Liquid C(CH_3)_3~+ Al_2Br_7~-

摘要

The room-temperature ionic liquid (RT-IL) C(CH_3)_3~+ Al_2Br_7~-(m.p. 2 ℃) 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 Br?nsted acid (protonated isobutene) with the potential for further reactivity. Based on the new absolute Br?nsted acidity scale, we calculated an absolute pH_(abs) value of 171 for liquid bulk C(CH_3)_3~+ Al_2Br_7~-. This value is about as acidic as 100 sulfuric acid (pH_(abs)= 171) and, thus, on the edge of superacidity.