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

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. When is an acid not an acid? Brønsted acidity is discussed for protic molecules in the sequence gas, liquid, liquid salt and liquid in solution. The individuality of the medium is paramount—acidic species can be cationic or anionic. Acidity in water cannot be used as a measure of acidity in ionic liquids; methods of measuring acidity are required for each system.