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

Abstract

In this study, the p K 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 N 10H reflected by thermodynamic parameters – Δ f ,298 H o, Δ 298 S o and Δ f ,298 G o – 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 p K 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.