Evaluation of Superacid Strength from the Protonation of Benzene. Comparison of HF-SbF5, HF-TaF5, and HBr-AlBr3 Systems

Article abstract of DOI:10.1021/jo00342a015

The degree of protonation of benzene in HF-SbF5, HF-TaF5, and HBr-AlBr3 solutions has been investigated by carbon-13 NMR spectroscopy.Both 30:1 HF-SbF5 and HBr-AlBr3 are much stronger acids than 30:1 HF-TaF5 (55percent protonation at a TaF5/benzene ratio of 3) or 4:1 HF-TaF5 (68percent protonation at a TaF5/benzene ratio of 2.5).HBr-AlBr3 protonates benzene completely down to a AlBr3/benzene ratio of 2.Under HBr pressure, 3.5-4 mol of HBr for each mole of AlBr3 are taken into the benzenium bromoaluminate solution.The "sludge" catalysts commonly encountered in hydrocarbon conversions are actually solutions of carbocations in the HBr-AlBr3 superacid.The high acidity revealed by the benzene protonation is representative for such sludges.The acidity measurement by benzene protonation can in principle be extended to other superacid systems.