Brønsted Acidity and the Medium: Fundamentals with a Focus on Ionic Liquids

Abstract

Fundamental aspects of Brønsted acidity in ionic liquid systems, in relation to those of simple protic molecules in the gas phase, pure protic molecules in the condensed phase and solutions of protic molecules in molecular systems, are presented. The variety of acidities possible, beyond those observed in aqueous systems, is emphasised and discussed in terms of differences of solvent levelling, ionisation, dissociation, homo-/hetero-conjugate ion speciation and the stabilisation of proton-transfer products from solvent to solvent. It is argued that data regarding aqueous systems do not necessarily explain acid/base behaviour in other liquids satisfactorily. Methods of measuring acidity are reviewed, particularly by spectrophotometry and electrochemistry and recommendations proffered for estimating speciation and acidity of ionic liquids of various complexities.

Brønsted acids in ionic liquids: how acidity depends on the liquid structure

“…22 Establishing pH or pK a in ionic liquid media is possible, albeit challenging, as it requires specialised equipment. 23 Hammett acidities reported vary widely depending on the measurement procedure used, 24,25 and the measurements are extremely sensitive to impurities, as they rely on UV-VIS probes. 26 A range of methodologies, associated difficulties, and best recommended practice have been expertly discussed by Johnson and co-workers.…”

“…26 A range of methodologies, associated difficulties, and best recommended practice have been expertly discussed by Johnson and co-workers. 24,25 A noteworthy addition to this strand of research is the use of an NMR probe to measure Hammett acidity, very recently introduced in ionic liquids by Welton and co-workers. 27 In this work, a Guttmann Acceptor Number (AN) 28 approach has been employed, which benefits from straightforward NMR-based methodology, and in the literature good correlations are found for measurements performed independently by different groups.…”

Aqueous Brønsted–Lowry Chemistry of Ionic Liquid Ions

“…[33] Cation formationr esultsi na nN ÀH + bond with 37 % s character,w ith 50 %b eing the maximum value for ap ure sp bond. Unfortunately,f ew reportsa re available that detail the significance of high temperature, non-aqueous stabilityo ft he NÀH + bond observed with high purity salts, which is easily captured in the typical variable temperature 1 HNMR experiment througho bservance of non-binomial coupling between 1 HÀ 14 N producing the 1:1:1p roton triplet resonance.…”

“…Ions of anhydrous protic liquid salts may possess drastically shifted pK values, and further differ from the analogous aqueous system in terms of speciation, stability and accompanying chemical equilibria. [33] 3-methyl-1H-imidazolium bromide, [HC 1 im]Br,p roduced from equivalents of dry hydrogen bromide (HBr) admitted directly to anhydrous liquid 1-methylimidazole (C 1 im) (Equilibrium 4, Ta ble 1) is an otable example. Both component molecules possess known BL acidbase character,w ell exploited in aqueous solution chemistry.I n the non-aqueous mixture, however,t he pure anhydrous salt that results possesses an on-acidic nitrogenic proton.…”

Absolute Brønsted Acidities and pH Scales in Ionic Liquids

“…[3,4] A general overview of the fundamental aspects of Brønsted acidity regarding practically all possible media including ionic liquids (ILs) was given by Mihichuk et al in their review in 2011. [5] Hammett acidity (H 0 ) measurements, [6] derived from protonation degrees of a series of indicator bases, have been established in ILs with dissolved Brønsted acids, for example, HCl [7] and HNTf 2 (Tf = trifluoromethanesulfonyl). [8,9] The range of acidities to be reached in IL medium is large, and for example, very acidic H 0 values down to À18 have been determined in acidic haloaluminate ILs.…”