Organic reactions are commonly performed in solution so that one of the most important parameter for the success ofa reaction is the choice of the “best” solvent. Indeed solvents may have a strong influence on reaction rates and equilibria. Solvent effects on the keto-enol tautomerism have been extensively studied. Recently, the equilibrium constants for the keto-enol interconversion of 2-nitrocyclohexanone (2-NCH) have been reported in water, some organic solvents and ionic liquids (ILs). Due to their peculiar properties, ILs may behave quite differently from conventional molecular solvents. According to these studies the enol form of 2-NCH appears to be predominant in apolar solvents while the keto form prevails upon transfer to water, aprotic polar organic solvents and ILs. In this work the temperature dependence of the tautomeric equilibrium constant for 2-NCH has been studied by UV-visible spectroscopy in five organic solvents, spanning a wide range of permittivity values, , their binary mixtures, and in some ILs in the temperature range 298 to 333 K. The thermodynamic parameters, G0, H0 and S0 , for the interconversion reaction have been derived. In the case of molecular solvents a good correlation was found between H0 and . An attempt to include the studied ILs in the same correlation fails. The nature of the ILs anion seems to play a fundamental role as H0 is < 0 for [TF2N]- based ILs while it is > 0 for [BF4]- based ILs.
Solvent effects on the keto-enol tautomerization reaction. A thermodynamic study in some organic solvents and ionic liquids
SIANI, Gabriella;ANGELINI, Guido;GASBARRI, Carla;FONTANA, Antonella;
2011-01-01
Abstract
Organic reactions are commonly performed in solution so that one of the most important parameter for the success ofa reaction is the choice of the “best” solvent. Indeed solvents may have a strong influence on reaction rates and equilibria. Solvent effects on the keto-enol tautomerism have been extensively studied. Recently, the equilibrium constants for the keto-enol interconversion of 2-nitrocyclohexanone (2-NCH) have been reported in water, some organic solvents and ionic liquids (ILs). Due to their peculiar properties, ILs may behave quite differently from conventional molecular solvents. According to these studies the enol form of 2-NCH appears to be predominant in apolar solvents while the keto form prevails upon transfer to water, aprotic polar organic solvents and ILs. In this work the temperature dependence of the tautomeric equilibrium constant for 2-NCH has been studied by UV-visible spectroscopy in five organic solvents, spanning a wide range of permittivity values, , their binary mixtures, and in some ILs in the temperature range 298 to 333 K. The thermodynamic parameters, G0, H0 and S0 , for the interconversion reaction have been derived. In the case of molecular solvents a good correlation was found between H0 and . An attempt to include the studied ILs in the same correlation fails. The nature of the ILs anion seems to play a fundamental role as H0 is < 0 for [TF2N]- based ILs while it is > 0 for [BF4]- based ILs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
