Photodecomposition of 4-amino-1,2,4-triazin-3,5-diones and -thiones in oxygenated aqueous solutions

Abstract The photodecomposition of newly synthesized 4-amino-1,2,4-triazinthione 1 and -dione 2 was examined in oxygenated aqueous solutions at various pH values. The degradation pathway of these compounds was compared with the photochemical decomposition of the herbicide metribuzin. Side-chain degradation by deamination, decarboxylation, sulfoxidation and dealkylation is typical for photolysis. Structurally similar compounds display substantial differences with respect to degradation. The pH value has an accelerating or inhibiting effect on the degradation rate. After the rapid formation of basic metabolites by the deamination of 1 and the decarboxylation of 2, selective decomposition takes place in alkaline and acidic solutions, whereas a non-selective course was detected under neutral conditions. The radicalic mechanisms of important reaction steps are discussed. Several metabolites with intact heterocyclic ring systems were identified as photochemically stable final products by the addition of a standard compound, such as 6-azauracile or 6-azathymine. In contrast to side-chain degradation, ring cleavage was only observed after long irradiation times. Carboxylic and ketocarboxylic acids as well as some inorganic ions were identified in the irradiated solution as the degradation products of side-chain degradation and ring cleavage.