Nitration of aromatic compounds with NO2/air catalyzed by sulfonic acid-functionalized ionic liquids

Nitration of aromatic compounds with NO2/air catalyzed by sulfonic acid-functionalized ionic liquid under solvent-free conditions is reported. A variety of ionic liquids [BSPy][HSO4], [BSPy][TfO], and [BSPy][pTSA] ([BSPy] = N-(4-hydroxsulfonylbutyl) pyridinium) were prepared. Their acidities were further investigated by Hammett method and the acidity-catalytic relationship was discussed. It was found that satisfactory results were obtained with [BSPy][HSO4]. Ionic liquids could be conveniently separated with the products and reused for five times with excellent yield of mono-nitration products and para-selectivity.