Parameters affecting mechanochemical nitration of aromatic precursors

Using MoO₃ as catalyst and NaNO₃ as nitronium source, aromatic precursors were nitrated in a planetary ball mill. Reaction times and nitrate/precursor molar ratios varied in the experiments. Nitration was observed with maximum yields approaching 80%. Product ratios of para to ortho isomers were consistently above 1, indicating reaction on the catalyst surface. Byproducts, including oxidation products and double nitration products, were observed. The product yield approached its theoretical maximum when the aromatic precursor was the limiting reactant; the yields remained well below their expected maxima when the limiting reactant was nitrate. Thus, the catalyst sites necessary to generate nitronium may be blocked by the excess of the aromatic precursor in such cases. Correlation of the reaction rates with characteristics of the precursors suggests the importance of both physical and chemical parameters. Factors affecting the reaction rate include the aromatic activation, basicity, enthalpy of vaporization, reaction enthalpies and viscosity.