Irradiation of an aerated bromacil (3‐sec‐butyl‐5‐bromo‐6‐methyluracil) aqueous solution (250 ppm), by exposure to direct solar radiation in the presence of different dye‐sensitizers and an appropriate pH, led to complete and fast photodecomposition of the title compound. The photoreaction mixture spotted on TLC showed one major intermediate, 3‐sec‐butyl‐2‐acetyl‐2‐hydroxy‐hydantoin. GLC showed another, minor, intermediate, the 5,5′‐dimer photo‐product of 3‐sec‐butyl‐6‐methyluracil. The most effective dye sensitizers checked were riboflavin (10 ppm) and methylene blue (2 to 5 ppm). Less effective were rose bengal (5 ppm), humic acids (20 ppm), and chlorophyll (20 ppm). The rate of reaction was pH dependent, being faster at alkaline pH and almost negligible in acidic solutions. Under optimal experimental conditions the photooxidation was completed after about 1 hour, and further irradiation resulted in the decomposition of the intermediate products and the formation of unidentified polar products.
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Acher et al. (1980) studied this question.