Protonation of Tetrakis(4-sulfonatophenyl)porphyrin in Aqueous Solutions of Acetonitrile and Dioxane

The protonation constants of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin were determined in binary mixtures of water with acetonitrile and 1,4-dioxane at 25 A degrees C and 0.1 mola <...dm(-3) sodium perchlorate. The pK (a) values decrease with addition of organic solvent to the media. The lack of a significant relationship between the protonation constants and dielectric constant of these solvent mixtures reveals the existence of specific solute-solvent interactions. Therefore the solvent effects were analyzed in terms of linear solvation energy relationships using the model of Kamlet, Abboud and Taft (KAT). Multiple linear regressions correlated the protonation constants with the microscopic parameters alpha (hydrogen-bond acidity), pi (au) (dipolarity/polarizability) and beta (hydrogen-bond basicity). In the all cases, an equation with two solvatochromic parameters gave the best correlations. For pK (1) values, the most significant parameters were found to be alpha and pi (au), whereas for pK (2) values, pi (au) and beta were the most predominant descriptors.