Aromatic nitration in aqueous nitric acid

Abstract

The rates of nitration of benzene, toluene, p-xylene, and mesitylene have been determined in aqueous nitric acid at concentrations between 24–41 mol % HNO₃ and at temperatures between 293–333 K. Plots of log₁₀(k₁/^aHNO₃)versus–H₀, where k₁ is the pseudo-first-order rate constant, ^aHNO₃the activity of the nitric acid, and H₀ the Hammett acidity function, are linear, and confirm that the mechanism of nitration in aqueous nitric acid is similar to that in aqueous sulphuric acid. The slopes of such plots for p-xylene and mesitylene have values of ∼2.0 whereas for benzene, toluene, and aromatic quaternary ammonium ions, the slopes have values closer to 3.0. The conclusion is drawn that with p-xylene and mesitylene the nitration is diffusion-controlled, but not so with the others.

The nitration activation energy, E, varies with both substrate and acid composition. At a given acid composition E_benzene > E_toluene > E_p-xylene > E_mesitylene. As in aqueous sulphuric acid, E, for a given aromatic decreases as the acid concentration is increased. The relative rates of nitration (benzene:toluene: p-xylene: mesitylene) at ∼30 mol % HNO₃ and 25 °C are found to be 1:22:256:485. The relative rates, however, are not constant. They narrow as the temperature or acid concentration is raised. Finally it has been shown that the similarity between nitration in aqueous nitric acid and that in aqueous sulphuric acid extends to the reaction between nitronium ion and water. In both media this appears to involve two water molecules and not just one.