Photochemistry and Photoinduced Toxicity of Acifluorfen, a Diphenyl‐Ether Herbicide

Abstract

Photochemistry studies can be helpful in assessing the environmental fate of chemicals. Photochemical reactions lead to the formation of by-products that can exhibit different toxicological properties from the original compound. For this reason the photochemical behavior of the herbicide acifluorfen (5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid) in the presence of different solvents was studied. Photochemical reactions were carried out using a high-pressure mercury arc and a solar simulator. Kinetic parameters and quantum yields were determined. The identification of photoproducts was performed by mass spectrometry and [1H] nuclear magnetic resonance (NMR). Nitrofluorfen, hydroxy-nitrofluorfen, 2-chloro-4-(trifluoromethyl)phenol, 5-trifluoromethyl-5'-nitrodibenzofuran, and other derivatives were identified. The photochemical reactions were also carried out in the presence of either a singlet or a triplet quencher, and in the presence of either a radical initiator or a radical inhibitor. Substances used as inhibitors of the excited levels T1 and S1 showed that photodegradation of acifluorfen begins from a singlet state S1 through a pi,pi* transition. The role of free radicals in the photodegradation of acifluorfen was determined and a radical mechanism was proposed. Toxicity tests against Daphnia magna Strauss showed that acifluorfen was not toxic at a concentration of 0.1 mM; however, photoproducts formed after 36 h of UV exposure of the herbicide induced a remarkable toxicity to the test organism.

Direct photolysis mechanism of pesticides in water

“…4) Decarboxylation via α-cleavage is the main photoreaction of acifluorfen. The homolytic mechanism via S 1 was proposed in acetonitrile, 87) while the heterolytic mechanism via short-lived T 1 was considered in water. 88) Acaricide cyflumetofen was rapidly photodegraded to the decarboxylated derivative with carboxylation at the 2-position of the 4-tert-butylphenyl ring, 4) which may be accounted for by the successive homolytic α-cleavage and rearrangement.…”

“…118) Although several energy quenching and radical scavenging experiments of acifluorfen suggested homolytic C-O cleavage from S 1 in acetonitrile, 87) photo-induced base-catalyzed ether cleavage was proposed instead in water. 88) Therefore, the reaction mechanism seems to be dependent on the solvent and the pH.…”

First Photolysis of Benzidine Schiff Base in Non Aqueous Solvents

“…This means that the rate of photolysis of Schiff base increase with increasing the polarity of solvent and consistent with previous photolysis studies (S. K. Pramanikabc and S.Dasb, A. Bhattacharyyaa , 2008) . A possible explanation for the higher photolysis rate in polar solvents is due to the contribution of an oxygen independent mechanism which enhanced photolysis reaction (R. Kumar and M. Yusuf, 2009) and polar solvents would achieve better stabilization of the free ions (L. Scrano, 2002 ).…”

“…Photolysis conversion of the bischromones is found to be dependent upon the nature of the intermediate spacer and H-donating capability of the solvent. L. Scrano et al (L. Scrano et al, 2002) reported the direct photolysis of acifluorfen in different solvents (water, methanol, acetonitrile and n-hexan) by UV mercury lamp. On the other hand,J.…”

Cu-Catalyzed Arylation of Phenols: Synthesis of Sterically Hindered and Heteroaryl Diaryl Ethers

“…7-9 This motif also appears in biologically active natural products, notably in the mammalian hormone thyroxine 10 and the vancomycin family of antibiotics. 11 There has been recent interest in the synthesis of atropisomeric diaryl ethers 12,13 as these may have application as molecular gears.…”