Effect of metal nitrate on mechanochemical nitration of toluene

Abstract

Mechanochemical nitration of toluene was explored using a planetary mill and MoO₃ as catalyst. Different inorganic nitrate salts were used as the nitronium source. Nitration was carried out by initially milling the nitrate salt and the catalyst, and then adding toluene. The amount of nitrate salt used was systematically varied, while the amounts of both the catalyst and toluene were held constant. For most nitrates the greatest yield of mononitrotoluene was observed for conditions where the molar reactant ratio of NO₃⁻ to toluene was near 4. Lower yields were observed with either less or more of the nitrate, consistent with limitation by the reactant on one hand, and obscuration of suitable catalytic sites on the other. The observed ratios of para- to ortho-mononitrotoluene were above one, consistently with a mechanism involving nitronium ions positioned at the catalyst surface reacting with toluene directly. Different nitrates resulted in varying mononitrotoluene yields, with copper nitrate showing the highest, and potassium nitrate the lowest yield, respectively. The observed yields were found to correlate with the enthalpy of the bulk reaction forming mononitrotoluene and the hydroxide of the cation of the respective nitrate used.