Proton transfer reactions in Ionic Liquids

M. PAULA LONGINOTTI; PEDRO F. ARAMENDÍA; M. LAURA JAPAS

Boulder, Colorado, EEUU

Simposio; Seventh Symposium On Thermophysical Properties.; 2009

Ionic Liquids (ILs) have been recently subject to intense studies due to an increasing demand for replacing toxic, flammable, volatile liquids in industrial processes. Therefore, many organic reactions (e. g Diels- Alder, Friedel-Crafts acylation, osmylation, Heck coupling, etc) have been carried our in this group of solvents. However, proton transfer reactions in ILs have received littele attention . thomazeau et. al established a Bronsted acidity sacle of IL solvents by analyzing the absortion spectra of an acid-base indicator in mixtures of strong acids with ILs. Later, MacFarlane et al analyzed qualitatively the basicity of ILs anions by studying the absortion spectra of acid base indicators of different acidity constants. In spite of that, none of this authors analyzed the magnitude of proton transfer reactions in ILs in comparison to the observed bahavior in other solvents, e.g water.  In this work we study the the equilibrium constants of of proton transfer reactions in bmimBF4 (1-butyl-3-methylimidazolim tetrafluoroborate) by the analysis of the absortion spectra of a weak acid (bromophenol blue), whose protonated and unprotonated forms have distintive spectra. The obtained results are compared with the equilibrium constants determined in water an with previous qualitative determinations. It can be concluded that the magnitude of proton transfer reactions in LIs solvents differs by various orders of magnitude from that observed in water. Proton transfer reactions in ILs were also analysed by meassuring the absortion and fluorescence spectra of a photoacid (pyranine) whose acidity constant varies various orders of magnitude after photoexcitation.