Competitive oxidative cleavage and epoxidation of 4,4′-diaminostilbene-2,2′-disulfonic acid by Fe(iii) aqua complexes

Abstract

4,4′-Diaminostilbene-2,2′-disulfonic acid (DSD) has been investigated as a model compound of fluorescent whitening agents. The reaction of DSD with Fe(III) aqua complexes has been studied in the dark at room temperature: a fast degradation of DSD was observed under our experimental conditions. HPLC analysis was used to follow the kinetics of the redox reaction. The process involves epoxidation, leading to DSD epoxide (1), and the oxidative cleavage of the double bond, leading to 5-amino-2-formyl-benzenesulfonic acid (2) together with the reduction of Fe(III) into Fe(II). The dependence of the redox process on [Fe(H₂O)₅(OH)]²⁺ concentration indicates that the dominant reaction pathway is a reaction between DSD and the Fe(III) monomeric species, [Fe(H₂O)₅(OH)]²⁺. The complete degradation of DSD is observed with an excess of oxidant; the ratio [Fe(H₂O)₅(OH)]²⁺∶[DSD] has to be higher than 2. A mechanism giving rise to the two degradation products is proposed.