Green S as a prototype for an environmentally-degradable dye: the concept of a ‘green dye’ in future Green Chemistry

The aqueous oxidation of Green S dye by hydrogen peroxide includes two alternative pathways: nucleophilic attack on the central carbon at near-neutral and high pH and the formation of N-oxide in the region of lower pH. This is indicative of a pH switch between nucleophilic and electrophilic reaction mechanisms for H2O2 oxidation. The strong catalytic effect of chloride anion together with methyltrioxorhenium (MTO) on Green S oxidation by hydrogen peroxide has been observed, conforming to the staggered mechanism which involves the formation of free chlorine at a very low steady-state concentration. In terms of the observed first-order rate constants, the catalytic effect is estimated to be at least 2000 compared to the uncatalysed reaction, for an MTO concentration of 3.0 × 10−6 mol dm−3 and Cl− concentration of 0.10 mol dm−3 at pH 1.3.