Study of solvent composition effects on the protonation equilibria of various anilines by multiple linear regression and factor analysis applied to the correlation between protonation constants and solvatochromic parameters in ethanol-water mixed solvents

The stoichiometric protonation constants (logbeta) of aniline derivatives were determined potentiometrically over a wide range of solvent composition (0 - 0.74 mole fraction of ethanol). To explain the variation of the logbeta values obtained over the whole composition range studied, the quasi-lattice quasi-chemical theory of preferential solvation was applied. The results were discussed in terms of macroscopic properties of the mixed solvent and different microscopic parameters, such as the Kamlet - Taft solvatochromic parameters to identify the solvent characteristics affecting the logbeta values. Kamlet and Taft's general equation was reduced to two terms by using both multiple-linear regression analysis and combined factor analysis and target factor analysis in these mixtures: the independent term and the hydrogen-bond donating ability alpha (HBD), which is a solvatochromic parameter. Hammett's reaction constant for the protonation of anilines has been obtained for all the solvent mixtures and correlates well with alpha (HBD) of the solvent.