Pendimethalin decomposed readily when irradiated in methanol a t wavelengths X 3 250 nm to form a number of products. In addition to the N-dealkylated intermediates, the principal products were 2-methyl-4,6-dinitro-5-[ (1-ethylpropyl)amino] benzaldehyde and 2-methyl-4,6-dinitro-5-[ (1-ethylpropy1)-amino] benzyl alcohol. The decomposition at sunlight wavelengths (A Z 290 nm) in methanol is relatively slower and also leads to N-dealkylation and arylmethyl oxidation. Neither acid nor alkali has an effect on the rate of disappearance of pendimethalin. At acidic p H N-dealkylation takes place, whereas at alkaline pH one of the NO2 groups is replaced by an OH group.Pendimethalin [N-( l-ethylpropyl)-2,6-dinitro-3,4-xylidine, I (Figure l)], a representative member of a growing list of N-substituted 2,6-dinitroanilines, is a selective herbicide that controls most annual grasses and certain broad-leaved weeds in cotton, soybean, and other crops.Soil persistence studies (Savage and Jordan, 1980) of this chemical indicated relatively rapid breakdown (half-life 4-6 days), while Walker and Bond (1977) reported a longer persistence in soil (half-life 72-172 days) with the persistence related to the moisture, temperature, and organic matter of the soil. Earlier studies (Parochetti and Dec, 1978) of the photodecomposition of I in soil exposed to sunlight showed much lower conversion (9.9% in 7 days) than other dinitroaniline herbicides previously studied (Leitis and Crosby, 1974). Recently, a number of photoproducts were identified when I was irradiated in methanol for 48 h (A = 300-400 nm). Photodecomposition was reported to involve oxidative dealkylation, nitro reduction, and cyclization (Dureja and Walia, 1989). A chemical present in the environment may undergo direct and indirect photochemical transformations. T h e latter may include photosensitized degradation, oxygenation, and photoinduced transformation. Earlier studies (Parochetti and Dec, 1978;Dureja and Walia, 1989) on the photodecomposition of I (A 3 300 nm) in solution and adsorbed on soil probably involve indirect photochemical processes. T o our knowledge, very little is known about the photochemistry of I at its maximum UV absorption (A < 250 nm). In the present work a comparative study of the photodecomposition of I under artificial sunlight exposure (A 3 290 and 250 nm) in an organic solvent (MeOH) is reported. In addition t o this, the effect of p H on the rate of degradation of I at X 3 290 nm is also examined.
EXPERIMENTAL PROCEDURES
Materials.Pendimethalin was obtained from Riedel-deHaen, FRG, and was purified by repeated crystallization from methanol, mp 56 O C (99% HPLC pure). 6-Nitro-3,4-xylidine (97% purity) was provided by Aldrich Chemical Co. and was used for comparison without purification. All the solvents used were of AR grade.Apparatus. The rate of disappearance of the pesticide was measured by a HPLC, Gibson-Abimed Model 302, equipped with a Shandon reverse-phase column (RP-18,25 X 0.4 cm) and a UV detector set at 240 nm; methanol was used as the ...
