The MC activity coefficient function for acid–base equilibria. Part 4. Limitations of empirical relationships involving observed nitration rates and acidity functions

Abstract

The empirical linear relationships, for moderately concentrated aqueous sulphuric acid solutions, between the observed nitration rates of aromatic compounds and the H₀, H_R, and (H_R+ log a_w) functions, are discussed. In particular, their utilization as a criterion of mechanism is investigated. The rate profiles of nitration of halogenobenzenes, for which wide acidity ranges are experimentally available, are used in the analysis. The results obtained show the breakdown of previous relationships because of either their incompatibility with the mechanism of nitration (H₀ function) or the approximation intrinsic in H_R acidity function. The latter, however, should be consistent with the nitration model, providing slopes of unity are rigorously observed. A new experimental linear relationship, consistent with nitration through NO₂⁺ ions, via a Wheland intermediate, has been found. The extension of the M_C activity coefficient treatment to acid–base equilibria involved in the nitration is presented.