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 pKa 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 – Δf,298Ho, Δ298So and Δf,298Go – for compounds 1–4 has been estimated using AM1and 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 pKa 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.