Photodegradation of 2-mercaptobenzothiazole disulfide and related benzothiazoles

Benzothiazoles are heterocyclic compounds used predominantly as rubber vulcanization accelerators. The overall goal of our research was to investigate the photodegradation behavior of 2-mercaptobenzothiazole disulfide, its degradation product 2-mercaptobenzothiazole, and further degradation product benzothiazole. Modern analytical techniques were utilized to follow photodegradation process at arbitrary intervals. A chromatographic method using reverse phase liquid chromatography was developed for the separation of benzothiazoles in the irradiated mixture. Direct photolysis of benzothiazole and 2-mercaptobenzothiazole in methanol at 253.7 and 313 nm in the presence or absence of oxygen was investigated at first. Benzothiazole was found to undergo photodimerization into 2,2'-bibenzothiazole, and in the presence of oxygen to give two additional photoproducts - 2-hydroxybenzothiazole and 2-methylbenzothiazole. The major degradation products of 2-mercaptobenzothiazole are benzothiazole and 2-benzothiazolesulfonic acid, with 2,2'-thiobisbenzothiazole, 2,x'-thiobisbenzothiazole (x = 4, 5, 6, 7), and 2-mercaptobenzothiazole disulfide as the minor degradation products. Direct photolysis of 2-mercaptobenzothiazole disulfide was investigated in four different solvents, at two different wavelengths (253.7 and 313 nm) and concentrations in the presence or absence of oxygen. In all cases 2-mercaptobenzothiazole and 2,x'-thiobisbenzothiazole were detected as the degradation products and in acetonitrile 2-thiocyanatobenzothiazole was also detected. A mechanism is proposed to rationalize the formation of photodegradation products. The effects of solvent, irradiation wavelength, and duration of irradiation time, concentration of the starting material and presence or absence of oxygen are summarized as well. It was observed that photodecomposition at 253.7 nm occurred at a much faster rate than at 313 nm and that less concentrated solutions decomposed faster. At higher concentration of 2-mercaptobenzothiazole its disulfide was detected as one of the degradation products. Methylated products were detected in methanol and acetonitrile and photoreaction took longer in polar protic solvents. Oxygenated products were formed in presence of oxygen and the photoreaction was slower as well in comparison to degassed solutions.