Thermodynamic nitration rates of aromatic compounds. Part 3. Nitration of aromatic compounds in concentrated aqueous trifluoromethanesulphonic acid

Abstract

The kinetics of nitration of mesitylene, toluene, benzene, bromobenzene and methyl phenyl sulphone, reacting as free bases in the range 50–100 wt% CF₃SO₃H are reported. The rates, related both to the stoichiometric concentration of nitric acid (rate =k_2obs[Ar][HNO₃]) and to the effective concentration of electrophilic species (rate =k₂°[Ar][NO₂⁺]) have been determined. Compared with k_2obs, which exhibits a large solvent effect, the true nitration rates (k₂°) were found to be almost independent of medium acidity and, for a given compound, rather similar in different acids. The k₂° values were also found to be linearly related to the rate constants for nitration in the gas phase.

The rates of reaction for methyl phenyl sulphone in 80–100 wt% acid were found to be lower in CF₃SO₃H than in H₂SO₄ by a factor of 8. A similar rate profile is observed, however, with an increase up to 90% acid and a decrease between 90 and 100%. The dependence of the rate profiles upon acidity and temperature in very concentrated acid solutions is discussed.