Kinetic investigation of the oxidation of substituted arylazonaphthol dyes by hydrogen peroxide in alkaline solution

Abstract

A number of arylazonaphthol dyes, namely substituted 2-arylazo-1-hydroxynaphthalene-3,6-disulfonates (1) and 1-arylazo-2-hydroxynaphthalene-6-sulfonates (2), have been synthesised and characterised by a range of techniques. All the dyes were found to exist predominantly in the hydrazone tautomeric form in aqueous media. Ortho-substituents on the aryl ring were found to increase dye pK_A values—irrespective of whether they are electron-withdrawing or -releasing—and the factors influencing pK_A values are discussed. Kinetic investigations have been made of the two dye series to pinpoint the active dye site and to identify the reactive species. In contrast to oxidation by HOCl and peracids, ortho substituents in the aryl ring of the 1-arylazo-2-naphthol dyes gave higher observed second-order rate constants, k_2obs, than corresponding para substituents; furthermore, k_2obs is not suppressed as the substituent increases in size or becomes charged. Ortho- and para-substituents gave similar rate constants, k₂^N, for nucleophilic reaction between the perhydroxy anion and the hydrazone tautomer and both gave good Hammett plots with slope ≈1. Values of k₂^N for the 2-arylazo-1-naphthol dyes (10^–3–10^–4 M^–1 s^–1) were over an order of magnitude less than the corresponding values for substituted 1-arylazo-2-naphthol dyes (10^–2 M^–1 s^–1). Product analysis was carried out for dye 2 (X = H), and a reaction mechanism has been proposed which involves reaction of the perhydroxy anion at the imine carbon of the hydrazone tautomer, producing an unstable diazene intermediate which decomposes liberating nitrogen to form benzene.