The base-catalyzed keto-enol interconversion of 2-nitrocyclohexanone in ionic liquids.

While the solvent effect on the rate of tautomerization of 2-nitrocyclohexanone in several organic solvents appears to depend essentially on the permittivity of the solvent, a different behavior is observed in some ionic liquids. In particular the rate of the reaction in ionic liquids (ILs) is much faster than expected solely on the basis of the permittivity of the studied ILs. However, if more solvent parameters are taken into consideration (namely the polarizability, H-bond acidity, and cohesive pressure of the solvent) one comprehensive linear solvation energy relationship (LSER) for both organic and IL media can be obtained and no special "ionic liquid effect" can be highlighted. The kinetic results have also been discussed in terms of the Brønsted equation. It turns out that on passing from molecular solvents to ILs the transition state structure of the enolization reaction becomes markedly enolate-like (Brønsted beta value close to 1).