Acidity In Ionic Liquids

Abstract:

Bronsted acids are formed in ionic liquids by the introduction of a protic molecule to a liquid salt or by reaction of it with an appropriate neutral base (e.g. HCl + pyridinium chloride, H2SO4 + 1-methylimidazole).The mixture of the pure protic acid HA, with pure base B, is often gauged for its ability to react to completion, thus forming the pure liquid salt [BH]+[A]-, from the behaviour of HA and B in water. While aqueous acid-base solutions are often the starting point of any discourse regarding acidity in general, we must realise that water is an atypical solvent. In this way, water is unsuitable as the universal acid-base reference system, since the varieties of acidities possible in non-aqueous media, either cannot form, or, are unstable, in water, due to the levelling effects of [H3O]+ or [HO]-.Indeed, acid speciation outside the realm of aqueous solution chemistry is rather varied and much more complex than that available in water, where homo- and hetero-conjugate ion speciation (e.g.[HCl2]-, [H2F3]+ and [HCl (AlCl4)]- ) can occur. Furthermopre, these species can form on either side of full 1:1 (or 1:2 etc) equivalence, where quantities of unreacted HA can be consumed by [A]- formed post proton-transfer, prior to consumption by unreacted base B.The current situation for acid/base chemistry in ionic liquids needs much more attention, with actual acidities determined on a per system basis: this can be done using available spectrophotometric and electrochemical methods. We present the wide world of acidity beyond that available in water, from the gas phase to the solvated proton under various constraints, and emphasize the differences in solvent levelling, ionisation, dissociation, ion speciation and the stabilisation of proton transfer products, for various acids in various media ( proton affinities and pK values are included for protic molecules in various solvents).

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