To estimate the reactivity of organic pesticides toward hydroxyl, a standard competitor, p-nitroso-N,N-dimethylaniline (PNDA), competes with a particular compound for the radicals produced by the photolysis of hydrogen peroxide. The rate at which the pesticide competes with PNDA for hydroxyl determines its reactivity. Of 10 pesticides tested, carbaryl was the most reactive, carbofuran and MCPA were of intermediate reactivity, and hexazinone was one of the least reactive. Experiments in irradiated field water, both in the laboratory and in sunlit rice paddies, showed that pesticide dissipation rates followed the order of reactivity with hydroxyl radical. Keywords: Hydroxyl radical; indirect photolysis; pesticides; aquatic fate
Trifluralin decomposed readily in water or aqueous methanol at sunlight wavelengths to form a multitude of products. In addition to the minor dealkylated intermediates, the principal product under acidic conditions was 2-amino-6-nitroo,n,«-trifluoro-p-toluidine. At alkaline pH, 2ethyl-7-nitro-5-trifluoromethylbenzimidazole represented about 80% of the photolysis products within 24 hr. Under all conditions, the highly polar 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-trifluoromethylbenzimidazoline and 2-ethyl-7-nitro-5-trifluoromethylbenzimidazole 3-oxide were present in significant amounts but were degraded by heat or further irradiation. The photochemical formation of benzimidazolines, benzimidazoles, and benzimidazole TV-oxides conforms to a general mechanism which should apply to many dinitroaniline herbicides.Trifluralin (2,6-dinitro-M Ar-dipropyl-a,a,«-trifluoro-ptoiuidine, I) is a selective preemergence herbicide representative of a growing list of N-substituted 2,6-dinitroanilines important to weed control. In 1970, 241,000 lb of trifluralin was used in California, while cotton and tomatoes alone employed almost 100,000 lb there in 1972 (California Department of Agriculture, 1970Agriculture, ,1972.The dinitroanilines are very unstable toward ultraviolet (uv) light. For example, ten products were detected by gas-liquid chromatography (glc) when I was irradiated in anhydrous methanol (Day, 1963), and 2,6-dinitro-Ar-n-propyl-rt,n,«-trifluoro-p-toluidine (VI) and 2,6-dinitro-u,«,«trifluoro-p-toluidine (XII) were tentatively identified by matching retention times. Irradiation of VI in heptane with a high-pressure mercury arc lamp (McMahon, 1966) in turn provided 2-nitro-6-nitroso-«,rt,o-trifluoro-p-toluidine. Exposure of I in hexane or methanol to laboratory uv light for 24 hr (Harrison and Anderson, 1970) gave products tentatively identified as VI, 2-amino-6-nitro-JV-propyl-«,n,o-trifluoro-p-toluidine, and 2-amino-6-nitro-MNdipropyl-u,n,n-trifluoro-p-toluidine. The products from the photodecomposition of I on glass or soil (Wright and Warren, 1965) or in aqueous acetone (Messersmith et ai, 1971) remained unidentified.The outdoor application and subsequent movement of I can result in exposure to sunlight, especially where the herbicide might occur in irrigation water, and preliminary experiments under these conditions indeed revealed rapid photodecomposition to a large array of substances (Leitis and Crosby, 1972). The purpose of the present work was to elucidate the products and pathways of this photodecomposition in sunlight and to positively identify the major products formed by uv irradiation of I in both aqueous and organic media. EXPERIMENTAL SECTIONSynthesis and Purification of Standards. Technical trifluralin (95%) (Eli Lilly and Co., Indianapolis, Ind.) was recrystallized from absolute ethanol until homogeneous on thin-layer chromatography (tic) and glc, mp 47-48°(48.5-49°, Probst et ai, 1967). 4-Chloro-3,5-dinitro-, , -trifluorotoluene (Eli Lilly) was recrystallized from ethanol, mp 55-57°(56-58°, Ha...
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