Hydration of methylimidazolium cations in aqueous solutions induced by UV radiation.

GABRIELA SARMIENTO; NICOLAS KRIMER; MARTÍN MIRENDA

Carlos Paz

Conferencia; 13th LATIN AMERICAN CONFERENCE ON PHYSICAL ORGANIC CHEMISTRY; 2015

INFIQC

Methylimidazolium cations are one of the most common constituents of Ionic liquids (ILs), a newfangled kind of organic salts that melt near or below 100 ºC. The distinctive physicochemical properties of the ILs compared to usual organic solvents - low flammability, moderate electrical conductivity and negligible vapor pressure, among others - make them interesting compounds for applications in different fields of science and technology. One important subject that remains to be solved at the moment concerns the degradation of methylimidazolium cations, since they are not readily biodegradable. We focus our work on the study of the photochemical degradation of methylimidazolium compounds induced by UV radiation in aqueous medium. Aqueous solutions of ~ 10-2 M of methylimidazolium compounds with different alkyl chains (Butyl, Ethyl and Methyl) were irradiated with a deuterium lamp at 210 nm for 24 hours. The products were separated from the reaction medium and analyzed by HPLC-PDA-ESI-MS. The isolated compounds were frozen, freeze-dried and subsequently analyzed by 1H-NMR and HSQC. Comparison between mass spectra reveals that the m/z of the mean reaction products are related to the m/z of the reactants by: m/zprod = m/zreact + 18. This result strongly suggests the addition of a water molecule to the methylimidazolium cation. All products showed similar UV spectra with λmax = 230 nm which, in turn, are very different to the absorption spectra of the reagents. Analysis of 1H-NMR spectra of the mixtures, before and after irradiation, shows that the products formed by addition of water to the methylimidazolium ring represent about a 50% of the total reaction products. There are other different products but, due to their lower concentrations and complex structures, their analysis and identification is beyond the scope of this study. The chemical structures of the mean reaction products were confirmed by the analysis of the 1H-NMR and HSQC spectra. The attachments of the hydroxyl group and the proton may occur either at positions 4 or 5, leading to positional isomers, according to the following scheme:The addition of the hydroxyl group is preferred at the ring side having the shorter and less bulky substituent. The ratio between isomers is 1.50 for the products with R = But, whereas falls down to 1.12 for the products with R = Et.Methylimidazolium D2O solutions were irradiated to determine if the addition of the hydroxyl and proton proceeds in a "syn" or "anti" conformation. For R ≠ Me, the addition of D and OD to the methylimidazolium ring yields eight different isomers. The disappearance of the "gem" coupling constant (J=14) in these experiments compared with those obtained in H2O solutions confirms that the addition of deuterium atom always occurs on the non-hydroxylated carbon. The analysis of the 1HNMR signals corresponding to the products obtained in D2O reveals equal amounts of "syn" and "anti" isomers.