Thermodynamic Analysis of Anion and Cation Effects on the Keto–Enol Equilibrium in Ionic Liquids. A Comparative Study with Conventional Solvents

Abstract

A comparative thermodynamic investigation of the keto-enol interconversion reaction has been performed in several organic solvents and room-temperature ionic liquids (RTILs) to evaluate the role of the solvent and the effect of the ionic composition of RTILs. The tautomeric constant (KT) values at different temperatures have been analyzed in terms of the van't Hoff relationship to give the relevant thermodynamic parameters. The ΔG° values are the results of quite different combinations of the ΔH° and ΔS° values depending on the nature of the solvent. As expected, in conventional solvents, the tautomeric equilibrium is enthalpically disfavored and entropically favored by the increase in solvent polarity. In ionic liquids, the nature of the anion seems to play a primary role in the thermodynamics of the reaction that is endothermic and enthalpically driven in PF6- and TF2N-based RTILs and exothermic but entropically driven in BF4-based RTILs. The cation effect on the thermodynamics of the reaction is more complex and is consistent with a prevalence of the alkyl side chain segregation in the organization of the ILs.

Properties of Apolar Solutes in Alkyl Imidazolium‐Based Ionic Liquids: The Importance of Local Interactions

“…A successful route towards gaining a deeper understanding is through the study of ILs with different compositions. A lot of effort has been made to study cation behavior, whereas recent studies of anion effects also reveal distinct solvation properties . In brief, some results indicated that apolar solutes such as benzene are better dissolved in ILs that have larger anions such as bis(trifluoromethanesulfonyl)imide (TFSI) or smaller cation/anion size ratios .…”

“…Al ot of effort has been made to study cation behavior, [28][29][30][31][32][33][34][35] whereas recent studies of anion effects also reveal distinct solvationp roperties. [21,[36][37][38][39][40][41][42][43][44][45][46][47][48] In brief, some resultsi ndicated that apolar solutes such as benzene are better dissolved in ILs that have larger anions such as bis(trifluoromethanesulfonyl)imide (TFSI) or smaller cation/ anion size ratios. [38] Nevertheless, it is not clear if this finding is of general validity or related to the properties of the molecular solute.…”

Ionic Liquids as “Masking” Solvents of the Relative Strength of Bases in Proton Transfer Reactions

“…Data collected in ILs seem to indicate that the thermodynamics of the above reaction is influenced mainly by the nature of the IL ions, with the anion playing a major role, as well as by the alkyl chain segregation in the heterogeneous organisation of ILs . Moreover, the reaction is faster in ILs than in conventional organic solvents, with rate constants higher than those expected on the basis of the p K a values .…”

“…[29] Data collected in ILs seem to indicate that the thermodynamics of the above reactioni si nfluencedm ainly by the nature of the IL ions, with the anion playing am ajor role, as well as by the alkyl chain segregationint he heterogeneouso rganisation of ILs. [25] Moreover, the reactioni sf aster in ILs than in conventional organic solvents, with rate constantsh igher than those expected on the basis of the pK a values. [26] This result could suggest ah igher basicity of the catalysti nI Ls than in conventional organic solvents but we werea ware that the pK a values collected in aqueous solution do not adequately describe the acid-base strength of speciesi nI Ls, [22,30] due to the peculiar IL-solute interactions.…”

“…We have investigated the effect of ILs on the thermodynamics and the kinetics of the base‐catalysed tautomerisation of 2‐nitrocyclohexanone. This reaction, which has been largely studied in water and in conventional solvents, has been shown to be a suitable probe for investigating the effects of ILs at the molecular level …”