Thermodynamic nitration rates of aromatic compounds. Part 4. Temperature dependence in sulfuric acid of HNO3→NO2+ equilibrium, nitration rates and acidic properties of the solvent†

Abstract

The protonation–dehydration equilibrium of nitric acid (HNO₃ + H⁺ ⇌ H₂O + NO₂⁺) has been studied in the range 80–94 wt% H₂SO₄ as a function of temperature and the variations of [NO₂⁺]/[HNO₃] ratios have been analysed to evaluate the dissociation constants (pK_NO2⁺) and the enthalpy of dissociation of the process.

The nitration rates of some aromatic compounds, 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 investigated in the range 50–100 wt% H₂SO₄ and the dependence upon acidity and temperature of the different rate profiles is discussed.

Activation energies determined by k_2obs and k₂° nitration rates are reported. The k₂° values are found to be independent of medium acidity and allow one to obtain consistent activation energies as expected for activated and deactivated aromatic substrates as well as for an encounter limited reaction.

Equilibria and rates in sulfuric acid have been analysed using the Mc activity coefficient functions whose values have been estimated over a range of temperature between 25 °C and 90 °C. The general trend observed in the acidic properties of the medium is reported. It is shown that the temperature has a small influence on the Mc function.