Photosensitized decomposition of terbacil in aqueous solutions

Abstract

The dye-sensitized photolysis of aerated aqueous solutions of terbacil (3-tert-5-chloro-6-methyluracil, I) exposed to solar irradiation was investigated under different reaction conditions. After 2 h of exposure to sunlight, in the presence of methylene blue (3 ppm) or riboflavin (10 ppm), the major photodegradation product in the pH range of 3.0-9.2 was 3-tert-butyl-5-acetyl-5-hydroxyhydantoin (11). In alkaline reaction mixtures (pH 8.0 or 9.2), this product undergoes deacylation, giving 3-tert-5-hydroxyhydantoin (111). In experiments carried out under neutral or acidic conditions (PH 6.8 or 3.0) with riboflavin, two additional products were isolated a mono-N-dealkylated terbacil dimer (IV) and a nonidentified water-insoluble product (V). The structures of 11,111, and IV were determined by using proton and carbon-13 nuclear magnetic resonance, infrared, ultraviolet, and mass spectroscopy.Terbacil (3-tert-butyl-5-chloro-6-methyluracil,

Reviews of Environmental Contamination and Toxicology

“…Thus, at basic or slightly basic pH, both MB and RF gave similar products as the major photoproducts, although at somewhat different rates. The major product of terbacil in all these experiments was 3-tert-butyl-5-acetyl-5-hydroxyhydantoin (147) (Reaction 29) (Acher et al 1981 ).…”

“…Terbacil (3-tert-butyl-5-chloro-6-methyluracil) (146) is another uracil herbicide, widely used for the selective control ofmany annual weeds in orchards and fields. Acher et al (1981) investigated the photolysis of dye-sensitized aerated aqueous solutions ofterbacil (146) under various reaction conditions. Aqueous solutions (250 ppm of 146) with MB (3 ppm) or RF (10 ppm) held in Pyrex test tubes were exposed to direct solar irradiations (1950 J!einsteins m -2S-1) under continuous aeration.…”

The role of indirect photochemical degradation in the environmental fate of pesticides: a review

“…The uracil herbicides bromacil and terbacil are stable in sunlight in the absence of sensitizers. 366,381,382 Although the addition of humic acids had no sensitizing effect on terbacil, the 1 O 2 sensitizers Rose Bengal and methylene blue did lead to indirect photodegradation of the herbicide. 382 While indirect photolysis of bromacil in the presence of hydrogen peroxide 366 or methylene blue 380,381 is possible, the ability of DOM to sensitize the photodegradation of the herbicide has not been investigated.…”

Surfactant-Altered Rates of Chlorimuron and Metsulfuron Photolysis in Sunlight

“…Sensitized photolysis can only occur if the compound acting as a photosensitizer has a higher triplet energy state than the secondary reactant (e.g., the herbicide molecule) (4). Direct photolysis of many herbicides can occur (5, 6, 7, 8, 10) and evidence of indirect herbicide photolysis has been reported (1,3,8,10,15).…”