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Ionic Liquids as “Masking” Solvents of the Relative Strength of Bases in Proton Transfer Reactions
Author(s) -
Zappacosta Romina,
Di Crescenzo Antonello,
Ettorre Valeria,
Fontana Antonella,
Pierini Marco,
Siani Gabriella
Publication year - 2018
Publication title -
chempluschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.801
H-Index - 61
ISSN - 2192-6506
DOI - 10.1002/cplu.201700514
Subject(s) - chemistry , ionic strength , proton , ionic liquid , equilibrium constant , ion , solvent , substituent , inorganic chemistry , thermodynamics , computational chemistry , organic chemistry , aqueous solution , physics , quantum mechanics , catalysis
Equilibrium constants for the proton transfer reaction between pyridines and trifluoroacetic acid were measured in room‐temperature ionic liquids (ILs) of different cation–anion compositions. The experimental equilibrium constants for ion‐pair formation were corrected according to the Fuoss equation. The calculated equilibrium constants for the formation of free ions were taken as a quantitative measure of the base strength in IL solutions and compared with the relative constants in water. The effect of IL composition is discussed for a series of fixed IL anions and fixed IL cations. Finally, the sensitivity of the proton transfer reaction to the electronic effects of the substituent groups on the pyridine ring was quantified by applying the Hammett equation. A more marked levelling effect on the base strength was observed in ILs than in water. The Hammett reaction constants ρ were then correlated with solvent parameters according to a multi‐parametric analysis, which showed that both specific hydrogen‐bond donor/acceptor and non‐specific interactions play an important role, with α and permittivity being the main parameters affecting the ability of the IL to differentiate the strength of the base.