Ionic liquids Hammett acidity

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Ionic liquids containing chloroaluminate ions are strong Lewis, Franklin and Bronsted acids. Protons present in [emim][AlCl4] have been shown to be superacidic with Hammett acidities up to —18. Such highly acidic ionic liquids are, nonetheless, easily handled and offer potential as non-volatile replacements for hazardous acids such as HF in several acid-catalyzed reactions. 

Pure chloroaluminate based ionic liquids have no proton donating ability. However, these liquids can demonstrate Bronsted acidity as a result of intentional or unintentional protonic species. These protons behave as superacids with Hammett acidity functions ranging from —12.6 to —18. The proton speciation in acidic... 

Imidazolium ionic liquids with a SO3H group appended to the imidazole nitrogen have been developed as promoters of the hydration of alkynes RC=CR to produce the corresponding ketones RCOCH2R. The Hammett acidity order, determined by UV-visible spectroscopy, has been found to be consistent with the activity. ... 

Hydrohalides dissolve easily in water to form hydrohalic acids and undergo virtually complete ionic dissociation (equation 84). All the hydrohalic acids, except HF, are very strong acids. Although AHF is one of the most acidic liquids, as measured by its Hammett function, hydrofluoric acid is a weak acid. 

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