Photodegradation of bromacil and terbacil by ultraviolet radiation

The photodegradation of bromacil (5-bromo-3-sec-butyl-6-methyluracil) and terbacil (5-chloro-3-tert-butyl-6-methyluracil) by ultraviolet radiation has been investigated. Irradiation of aerated aqueous solutions of the title compounds (25 ppm), led to fast degradation of both. The reactions were found to follow first order kinetics with rate constants 5.77x10^-1 hr^-1 in the case of bromacil and 8.45x10^-2 hr^-1 in the case of terbacil. Degradation in ethanol and cyclohexane was also studied, but the solutions were not analyzed for photoproducts. The photoreaction mixture of both the herbicides in water showed four peaks on GC, indicating four photoproducts. Loss of halogens from the heterocyclic ring has been observed as shown by the presence of Br- in the case of bromacil and Cl- in the case of terbacil in the photoreaction solution. 6-Methyluracil which was isolated from the aqueous fraction of the photoreaction mixture was the common photoproduct from both bromacil and terbacil. From the organic fractions 3-sec-butyl-5-acetyl-5-hydroxyhydantoin was identified in the case of bromacil and 3-tert-butyl-5-acetyl-5-hydroxyhydantoin in the case of terbacil. The other two photoproducts were left unidentified.