Kinetic investigations of the oxidation of arylazonaphthol dyes in hypochlorite solutions as a function of pH

Abstract

A kinetic investigation has been made of the oxidation of 4-(4-sulfophenylazo)-1-naphthol (Orange I, 1), 1-(4-sulfophenylazo)-2-naphthol (Orange II, 2) and a series of substituted arylazonaphthol dyes in hypochlorite solution as a function of pH. Two chemically distinct reactions occur, one at high pH and the other in acid media (pH < 5), and both involve electrophilic oxidation of the nitrogen atom adjacent to the aryl ring. In acid media, aqueous chlorine reacts with the N–H group of the hydrazone tautomer (k₂ ≈ 10² M^–1 s^–1); in alkaline media, reaction of HOCl with the common anion is much faster (k₂ ≈ 10⁵ M^–1 s^–1), due to delocalisation of the negative charge onto the nitrogen atom. Ortho- and para-substituents influence rate constants for both reactions, producing Hammett plots with slopes of –2 and –0.5 in acid and alkaline media, respectively. A mechanism for dye oxidation in alkaline media is proposed. Overall mechanistic pathways of dye degradation are extremely complex due to degradation of initial reaction products into small fragments. If an amino group is substituted into the dye, e.g. at the 8-position of 2-arylazo-1-naphthol dyes, rate constants for oxidation by aqueous chlorine increase by 500-fold. This indicates that direct reaction with the amino group becomes a precursor to degradation; this pathway is eliminated if an electron-withdrawing triazine group is introduced into the amino group.