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Aromatic compounds have been nitrated in high yields in ionic liquids by chemists at Imperial College London. Lecturer Llewellyn Lancaster and graduate student Verónica Llopis-Mestre employed acetyl nitrate, which is a fairly mild nitrating agent, and ionic liquids derived from imidazolium and pyrrolidinium cations to nitrate aromatic substrates [Chem. Commun., 2003, 2812]. They used an excess of substrate to nitrating agent and studied the extent of reaction under a range of conditions. "We achieved atom-efficient nitration, based on nitric acid, of a range of aromatic substrates," Lancaster tells C&EN. "The nitration results for chlorobenzene and bromobenzene are particularly pleasing because these yields are far better than those obtained using molecular solvents such as dichloromethane. The results suggest that the ionic liquid makes the acetyl nitrate system more reactive than it is in molecular solvents." The two chemists also showed that the best yields are obtained using 1-butyl-1-methylpyrrolidinium, an ionic liquid cation that is stable to nitration. They propose that acetyl nitrate completely dissociates in the ionic liquid to acetate and nitronium [NO2+], which is a potent nitrating agent. Lancaster is now investigating the mechanism of the reaction and plans to extend the study to other electrophilic reactions in ionic liquids
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