Protonation equilibria of 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes

Abstract

The spectroscopic behaviour of substituted 1-arylazo-4-naphthols and 1-arylazo-4-methoxynaphthalenes has been investigated in a solvent system consisting of 20% v/v ethanol and 80% 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, pkBH+ values have been calculated using Jaffé'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 pKBH+ values with σm and σp of the substituents are found, owing to the presence in the molecules of two distinct basic centres.