On the role of the solvent and substituent on the protonation equilibria of di-substituted anilines in dioxane-water mixed solvents

Protonation constants of a number of di-substituted anilines were determined potentiometrically in 0, 20, 30, 40, 50, and 60% (v/v) dioxane-water mixtures at (25.00 +/- 0.02) degrees C with an ionic strength of 0.10 mol-dm(-3) sodium perchlorate. The data are discussed in terms of the electronic character of the substituents. Two different methods were used to study the effects of the solvents on the protonation constants; one involved a single polarity parameter, the Dimroth-Reichardt parameter E-T(30); the other involved the Kamlet-Taft multi-parametric method. The protonation constants of di-substituted anilines correlate with the molecular parameters for the dipolarity/polarizability of the solvent, pi*, and its hydrogen-bond acceptor ability, beta.