Reaction patterns in photooxidative degradation of two herbicides

Different reaction patterns of photooxidation has been witnessed for the herbicides metribuzin, (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazine-5-one) and bromacil (5-bromo-3-sec-butyl-6-methyluracil). In the first case oxygen has a pronounced effect on the rate of photooxidation, while the influence of hydrogen peroxide is quite moderate. The photolytic process in this case would apparently start via a reaction of the excited herbicide molecule with hydrogen peroxide or with oxygen. In another case, exemplified by bromacil, oxygen does not have a pronounced effect on the rate of photooxidation, which however is considerably enhanced by hydrogen peroxide. The reaction is initiated by hydroxyl radicals generated by hydrogen peroxide photolysis. These conclusions are supported by the different effects of isopropanol inhibition.