Photodegradation pathways and mechanisms of the herbicide metamifop in a water/acetonitrile solution

The herbicide metamifop and related compounds were irradiated by UV (λ = 300 nm) for various time periods, and degradation products were analyzed by gas chromatography and gas chromatography-mass spectrometry to assess the fate of metamifop in the environment. Nearly 10% of metamifop degraded within 30 min of irradiation and >80% degraded after 4 h. Among the metabolites, N-(2-fluorophenyl)-2-hydroxy-N-methylpropionamide (HPFMA) formed at the highest level (50.8%), followed by N-methyl-2-fluoroaniline (NMFA, 8.5%), N-methyl-2-oxo-N-phenylpropionamide (MOPPA, 6.6%), N-(2-fluorophenyl)-2-(4-hydroxyphenoxy)-N-methylpropionamide (HPPFMA, 3.9%), and 4-(6-chlorobenzooxazol-2-yloxy)phenol (CBOP, 1.2%) after 4 h of irradiation. HPPFMA degraded further to yield HPFMA (32.5%). CBOP also degraded to give 6-chloro-3H-benzooxazol-2-one (CBO, 6.6%). It is proposed that homolytic fission of C-O bonds occurred at the early stage of photolysis and further reactions with a hydroxyl radical and/or a hydrogen radical formed various metabolites. Standards, which are not commercially available, were synthesized in the authors' laboratory.