Binary Systems of a Hydrophobic Aprotic Ionic Liquid and Water as Catalysts for Michael Addition Reaction

Abstract

Binary systems of an Aprotic Ionic Liquid (AIL), 8-hexyl-1,8-diazabicyclo[5.4.0]-undec-7-ene-8-iumhydroxide ([C6DBU]OH) and water were prepared at molar ratio, XAIL ranging from 0 to 1.0. Physicochemical properties of the pure and binary systems of the AIL have been studied in detail by viscosity, Fourier Transform Infrared (FTIR) spectroscopy, and dynamic light scattering measurements and thermogravimetric analysis. The negative deviation of excess viscosity at XAIL < 0.4 indicated the formation of micelle like aggregation and the positive deviation of excess viscosity at XAIL > 0.4 indicated the formation of reverse micelle like aggregation due to the surfactant-like behavior of the long alkyl chain in [C6DBU]OH. The spectral and the particle size analyses show the presence of the confined water at XAIL > 0.4 in the cored structure of the reverse micellar aggregates. The variation of the microstructures in water-rich and ionic liquid (IL)-rich region significantly influenced the kinetics of Michael addition reaction between acetylacetone and 2-cyclohexene-1-one in absence of organic solvents while using [C6DBU]OH and its binary systems with water as catalysts. The reaction was studied by using thin layer chromatographic technique using aluminum plates coated with silica gel as the stationary phase and mixture of chloroform and n-hexane (1:1 by volume) as the eluent. The progress of the addition reaction was monitored by observing the development of spots in the chromatographic plate. The kinetic investigations in the presence of 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU), NaOH, and a DBU based protic IL, [HDBU]OH have also been made and the catalytic performances have been compared. Finally, the role of the [C6DBU]OH and its binary systems with water as catalysts in the mechanism of the Michael addition reaction has been explained in terms of different molecular interactions.

Keywords: Aprotic ionic liquid, Average reaction rate, Catalyst, Micelle, Reverse Micelle, Michael addition reaction, 1, 8-diazabicyclo[5.4.0]-undec-7-ene (DBU).

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