Protonation Equilibria of 1-Arylazo-4-naphthols and 1-Arylazo-4-Methoxynaphthalenes

Article abstract of DOI:10.1039/P29800000628

The spectroscopic behaviour of substituted 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes has been investigated in a solvent system consisting of 20percent v/v ethanol and 80percent v/v sulphuric acid-water in order to measure the protonation equilibrium constants.Some naphthols (I; X=H, o-OCH3, p-OCH3, o-CH3, p-Cl) and all the methoxynaphthalenes (II) show two absorption bands due to the free base and the conjugate acid, with a characteristic isosbestic point.For these compounds, pK_BH(1+) values have been calculated using Jaffe's acidity function.Other naphthols (I; X=m-OCH3, m-CH3, p-CH3, o-Cl, m-Cl) show a single absorption band which shifts on changing the acidity of the medium.The lack of an isosbestic point has been ascribed to the overlap of the azo-hydrazone equilibrium with the protonation equilibrium of the azo-group.For methoxynaphthalenes, separate correlations of pK_BH(1+) values with ?_m and ?_p of the substituents are found, owing to the presence in the molecules of two distinct basic centres.

Full text of DOI:10.1039/P29800000628

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