Mechanochemical nitration of aromatic compounds

Nitration of the benzene ring plays a central role in the production of several important energetic compounds, such as TNT. The conventional nitrating agent for this reaction is a mixture of concentrated sulfuric and nitric acids, which is highly corrosive and requires special handling and disposal procedures. Application of mechanochemical methods to aromatic nitration can allow this synthesis, or at least some of its steps, to be carried out solvent-free, using reactants that are safer to handle and less harmful to the environment. Solid phase nitration of aromatic compounds was performed by ball milling the aromatic substrate with sodium nitrate and a Lewis acid catalyst. Naphthalene and toluene were used as aromatic compounds. Different Lewis acid catalysts were tested. The aromatic rings were successfully nitrated using AlCl₃ and MoO₃ as catalysts. High yields were observed with AlCl₃, although the reaction occurred with and without milling. With MoO₃, yields were small but showed strong dependence on the milling time and on the ball to powder mass ratio. Milling liquid toluene with NaNO₃ and MoO₃ at room temperature produced mononitrotoluenes in small yields and at prolonged milling times. The same reaction was carried out in solid phase under liquid nitrogen, however, the results were inconclusive. Other metal oxides were explored as catalysts as well, but yielded no nitrated products.