Basicity of Pyridine and Some Substituted Pyridines in Ionic Liquids

Abstract

The equilibrium constants for ion pair formation of some pyridines have been evaluated by spectrophotometric titration with trifluoroacetic acid in different ionic liquids. The basicity order is the same in ionic liquids and in water. The substituent effect on the equilibrium constant has been discussed in terms of the Hammett equation. Pyridine basicity appears to be less sensitive to the substituent effect in ionic liquids than in water.

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

“…Cheng et al. have measured the p K a values of a series of carbon acids, benzoic acids and sulfonamides in three bis(trifluoromethylsulfonyl)imide (triflimide, Tf 2 N)‐based ILs and in 1‐butyl‐3‐methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]), by using the overlapping indicator method and avoiding the previously suggested ion‐pairing effect in ILs . Indeed the carbanions of the C−H acids used as substrate or indicators are sterically hindered and largely charge‐delocalised, properties that reduce the occurrence of ion‐pair formation.…”

“…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 . This result could suggest a higher basicity of the catalyst in ILs than in conventional organic solvents but we were aware that the p K a values collected in aqueous solution do not adequately describe the acid–base strength of species in ILs, due to the peculiar IL–solute interactions.…”

“…We followed the suggestion that in ILs an acid–base reaction leads to the formation of ion pairs rather than free ions, especially if ions are not hindered and not largely charge‐delocalised. Indeed, the relevant mathematical treatment of experimental data, which takes into account the formation of ion pairs, has provided a good fit . The results obtained have shown that the basicity order of pyridines in [BMIM][PF 6 ] and water is the same, whereas the effects of structural changes of solutes are less significant in the IL than in water .…”

Reaction Mechanism in Ionic Liquids: Kinetics and Mechanism of the Aminolysis of 4-Nitrophenyl Acetate

“…On the other hand, Chiappe and collaborates have used a Brønsted‐type correlation to understand the mechanism of the base‐catalyzed enolization reaction of 2‐nitrocyclohexanone using substituted pyridines in several ILs. They found that the β values (β > 0.9) for this reaction agree with a mechanism via solvent‐stabilized enolate‐like transition state when ILs are the reaction media .…”

“…In nucleophilic substitutions reactions, the Brønsted equation (log k vs. p K a of amine) and particularly the charge developed on the nucleophilic atom from reactants to the transition state (the slope of the Brønsted‐type plot, β) has been a very good tool to determine reaction mechanisms in water and in organic solvents . There is a great deal of information in the literature about β values for these reactions in conventional solvents; nevertheless, the information is scarce for the same reactions in ILs .…”