Bromoxynil

Composition Tech. grade is c. 95% pure. Mol. wt. 276.9 M.f. C₇H₃Br₂NO Form Colourless solid. M.p. 194-195 ºC (sublimes at 135 ºC/0.15 mmHg); (tech., 183-192 ºC) V.p. 6.3 ´ 10^-3 mPa (20 ºC) K_OW logP = 2.8 (unionised) Henry 1.34 ´ 10^-5 Pa m³ mol^-1 (calc.) Solubility In water 130 mg/l (20 ºC). In dimethylformamide 610, tetrahydrofuran 410, acetone, cyclohexanone 170, methanol 90, ethanol 70, mineral oils <20, benzene 10 (all in g/l, 25 ºC). Stability Very stable to dilute alkalis and acids. Stable to u.v. light. Thermally stable below the melting point. pKa 3.86

COMMERCIALISATION Bromoxynil

History Herbicidal properties of bromoxynil described independently by R. L. Wain (Nature (London), 1963, 200, 28), by K. Carpenter & B. J. Heywood (ibid., p. 28), and by Amchem Products Inc. Development reviewed by B. J. Heywood (Chem. Ind. (London), 1966, p. 1946). Introduced by May & Baker Ltd and by Amchem Products Inc. (both now Aventis CropScience). Patents GB 1067033 to May & Baker; US 3397054; US 4332613 both to Amchem Manufacturers Aventis; CFPI Nufarm; Makhteshim-Agan; Punjab; Sanachem; Sundat; Zhejiang

APPLICATIONS Bromoxynil

Biochemistry Photosynthetic electron transport inhibitor at the photosystem II receptor site; also uncouples oxidative phosphorylation. Mode of action Selective contact herbicide with some systemic activity. Absorbed by the foliage, with limited translocation. Uses Post-emergence control of annual broad-leaved weeds, especially young seedlings of the Polygonaceae, Compositae, and certain Boraginaceae, in cereals, maize, sorghum, flax, onions, garlic, mint, grass-seed crops, turf, and non-crop land. Often used in combination with other herbicides, to extend the spectrum of control. Formulation types EC; SC; WP.

ANALYSIS Bromoxynil

Product analysis (octanoate) by glc (AOAC Methods, 1995, 980.05, CIPAC Handbook, 1985, 1C, 1989-1998) or by determination of bromine. Residues determined by glc of a derivative (H. S. Segal & M. L. Sutherland, Anal. Methods Pestic., Plant Growth Regul. Food Addit., 1967, 5, 347; Anal. Methods Pestic. Plant Growth Regul., 1972, 6, 605) or by i.r. spectrometry.

MAMMALIAN TOXICOLOGY Bromoxynil

Oral Acute oral LD₅₀ for rats 190, mice 110, rabbits 260, dogs c. 100 mg/kg. Skin and eye Acute percutaneous LD₅₀ for rats >2000, rabbits 3660 mg/kg. Mild eye irritant; non-irritating to skin (rabbits). Not a skin sensitiser (guinea pigs). Inhalation LC₅₀ (4 h) for rats 0.41 mg/ml. NOEL (2 y) for rats 100 ppm. Toxicity class WHO (a.i.) II; EPA (formulation) II EC hazard R63| T; R25

ECOTOXICOLOGY Bromoxynil

Birds Acute oral LD₅₀ for pheasants 50, hens 240, quail 100-125, mallard ducks 200 mg/kg. Sub-acute oral LC₅₀ (21 d) for pheasants 4000 ppm diet. Fish LC₅₀ (48 h) for goldfish 0.46, catfish 0.063 mg/l. Daphnia LC₅₀ (48 h) 12.5 mg/l. Algae EC₅₀ (72 h) for Scenedesmus subspicatus 140 mg/l. Bees LD₅₀ (48 h, oral) 4 mg/bee; harmful to bees.

ENVIRONMENTAL FATE Bromoxynil

Animals See plants. Plants Metabolism in plants and animals is by hydrolysis of the ester and nitrile groups, with some debromination occurring (J. H. Buckland, Pestic. Sci., 1973, 4, 149, 689). Soil/Environment In soil, DT₅₀ c. 10 d. Degraded by hydrolysis and debromination to less toxic substances such as hydroxybenzoic acid. For persistence in soil, see Pestic. Sci., 1980, 11, 341.

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