The role of the medium on the acid dissociation constants of some azo dyes in view of experimental and theoretical data

In this study, the pK(a) values of some azo dyes derived from cyclohexane-1,3-dione in different organic solvent + water mixtures have been determined spectrophotometrically. The organic solvents used are ethanol, acetone and N,N-dimethylformamide (DMF). Thus, the nature and the proportion of the organic cosolvent effects as well as the molecular structure on acidity constants were evaluated. Furthermore, the proton-donating ability of N10H reflected by thermodynamic parameters -Delta H-f,H-298 degrees, Delta S-298 degrees and Delta(f,298)G degrees - for compounds 1-4 has been estimated using AM1 and PM6 methods in the gas and aqueous phases. The data obtained provides a complete and accurate picture of the acid-base properties of the compounds under study. The pK(a), values predicted in aqueous phase by the previous methods were compared with the experimental values and the best agreement with the experimental data was obtained by AM1. (C) 2010 Elsevier B.V. All rights reserved.