Kinetic investigations of the oxidation of Methyl Orange and substituted arylazonaphthol dyes by peracids in aqueous solution

Abstract

Kinetic and product analyses have been made of the oxidation of an activated azo dye, 4-(4-sulfophenylazo)-N,N-dimethylaniline (Methyl Orange, 1) and a series of tautomeric arylazonaphthol dyes by peracids at 40 °C in alkaline media, particularly peroxosulfate. The arylazonaphthol dyes are readily degraded into fragments (k₂^E = 10²–10³M^–1 s^–1) whereas the azo link in Methyl Orange is relatively inert (k₂^E  10^–1M^–1 s^–1) consistent with the view that dyes that contain the azo or hydrazo link are more resistant to oxidation than the common anion form of arylazonaphthol dyes. Methyl Orange is decolourised by peroxosulfate (k₂^E = 7.6 M^–1 s^–1) simply via oxidation of the tertiary amine to amine oxide, resulting in a shift of the spectrum into the UV region. The reaction site in arylazonaphthol dyes has been pinpointed by investigating k₂^E for substituted dyes with the 1-arylazo-2-naphthol and 2-arylazo-1-naphthol structural motifs. In direct contrast to results for oxidation by hydrogen peroxide, the presence of an ortho-sulfonate in the aryl ring retards oxidation but an ortho-sulfonate in the naphthol ring does not. This suggests oxidation is initiated by electrophilic reaction by the negatively charged peroxosulfate at the nitrogen atom of the common dye anion that is in close proximity to the aryl ring. In contrast to oxidation by hypochlorite, the introduction of an amino group into the 8-position of 2-arylazo-1-naphthol dyes protects the dye against oxidation, via its influence upon the dye pK_A.