Simazine

NOMENCLATURE
Common name: simazine
IUPAC name: 6-chloro-N2,N4-diethyl-1,3,5-triazine-2,4-diamine
Chemical Abstracts name: 6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine
CAS RN: [122-34-9]

PHYSICAL CHEMISTRY
Composition: Tech. grade is 97% pure. Mol. wt.: 201.7; M.f.: C₇H₁₂ClN₅ Form: Colourless powder. M.p.: 225-227 ^oC(decomp.) V.p.: 2.94x10^-3 mPa (25 ^oC) (OECD 104). KOW: logP = 2.1 (25 ^oC, unionised). Henry: 5.6x10^-5 Pa m³ mol^-1 (calc.) S.g./density: 1.33 (22 ^oC). Solubility: In water 6.2 mg/l (pH 7, 20 ^oC). In ethanol 570, acetone 1500, toluene 130, n-octanol 390, n-hexane 3.1 (all in mg/l, 25 ^oC). Stability: Relatively stable in neutral, weakly acidic and weakly alkaline media. Rapidly hydrolysed by stronger acids and bases; DT₅₀ (calc.) 8.8 d (pH 1), 96 d (pH 5), 3.7 d (pH 13) (20 ^oC). Decomposed by u.v. irradiation (c. 90% in 96 h). pKa: 1.62 (20 ^oC), v. weak base.

APPLICATIONS
Biochemistry: Photosynthetic electron transport inhibitor at the photosystem II receptor site. Maize tolerance of triazines is attributed to conjugation with glutathione.
Mode of action: Selective systemic herbicide, absorbed principally through the roots, but also through the foliage, with translocation acropetally in the xylem, accumulating in the apical meristems and leaves.
Uses: Control of most germinating annual grasses and broad-leaved weeds in pome fruit, stone fruit, bush and cane fruit, citrus fruit, vines, strawberries, nuts, olives, pineapples, field beans, french beans, peas, maize, sweet corn, asparagus, hops, alfalfa, lupins, oilseed rape, artichokes, sugar cane, cocoa, coffee, rubber, oil palms, tea, turf and ornamentals. Applied at rates up to 1.5 kg/ha within the EU, and up to 2-3 kg/ha in perennial crops in the tropics and subtropics.
Phytotoxicity: Phytotoxic to a number of crops, including sugar beet, tobacco, tomatoes, cucurbits, clover, rice, soya beans, lettuce, oats, and many vegetables (e.g. spinach, onions, carrots, crucifers, etc.).
Formulation types: GR; SC; WG; WP.

MAMMALIAN TOXICOLOGY
Oral: Acute oral LD50 for rats 500-10 000, Chinese hamsters >5000 mg/kg.
Skin and eye: Acute percutaneous LD₅₀ for rats >2000 mg/kg. Non-irritating to skin and eyes (rabbits).
Inhalation: LC₅₀ (4 h) for rats >5.5 mg/l.
NOEL: (2 y) for female rats 0.5 mg/kg b.w. daily; (1 y) for female dogs 0.8 mg/kg b.w. daily; (95 w) for mice 5.7 mg/kg b.w. daily.
ADI: 0.005 mg/kg b.w.
Toxicity class: WHO (a.i.) III (Table 5); EPA (formulation) IV
EC hazard: R40

ECOTOXICOLOGY
Birds: Acute oral LD₅₀ for mallard ducks >2000 mg/kg. Dietary LC₅₀ (8 d) for mallard ducks 10 000, Japanese quail >5000 mg/kg.
Fish: LC₅₀ (96 h) for bluegill sunfish 90, rainbow trout >100, crucian carp >100, guppies 49 mg/l.
Daphnia: LC₅₀ (48 h) >100 mg/l; (21 d) 0.29 mg/l.
Algae: EC₅₀ (72 h) for Scenedesmus subspicatus 0.042 mg/l; (5 d) for Selenastrum capricornutum 0.26 mg/l.
Bees: LD₅₀ (48 h, oral and topical) >99 ug/bee.
Worms: LC₅₀ (14 d) for earthworms >1000 mg/kg.

ENVIRONMENTAL FATE
Animals: In mammals, following oral administration, 65-97% is eliminated within 24 h as the de-ethylated metabolite. Elimination of low rates is primarily in the urine of rats, with a shift to faecal elimination at high doses. Excretion is rapid (c. 90% in 48 h). Degradation to desethylsimazine and bis-desethylsimazine (diaminochlorotriazine) is the primary metabolic pathway.
Plants: Readily metabolised by tolerant plants to the herbicidally-inactive 6-hydroxy analogue and amino acid conjugates. The hydroxysimazine is further degraded by dealkylation of the side-chains and by hydrolysis of the resulting amino groups on the ring, with evolution of CO2. In sensitive plants, unaltered simazine leads to chlorosis and death.
Soil/Environment: Major metabolites under all conditions are desethylsimazine and hydroxysimazine. Microbial breakdown in soil results in degradation of simazine at very variable rates; DT₅₀ 27-102 d (median 49 d); temperature and soil moisture are the main factors affecting rates. Koc 103-277 (median 160); Kd 0.37-4.66 (12 soils). Under field conditions, simazine has a low leaching potential. Loss by direct photodecomposition is insignificant. Indirect photodecomposition in the presence of photosensitisers such as humic acids is, however, likely.