2.4-D.2,4-D-dimethylammonium.

NOMENCLATURE

2,4-D

Common name 2,4-D (BSI, E-ISO, (m) F-ISO, WSSA); 2,4-PA (JMAF)

IUPAC name (2,4-dichlorophenoxy)acetic acid 

Chemical Abstracts name (2,4-dichlorophenoxy)acetic acid 

CAS RN [94-75-7]  EEC no. 202-361-1  Development codes L208 (Marks)

PHYSICAL CHEMISTRY

2,4-D

Composition Tech. is ³96% pure.  Mol. wt. 221.0  M.f. C₈H₆Cl₂O₃  Form Colourless powder, with a slight phenolic odour.  M.p. 140.5 ºC  V.p. 1.86 ´ 10^-2 mPa (25 °C, OECD 104)  K_OW logP = 2.58-2.83 (pH 1), 0.04-0.33 (pH 5)  Henry 1.32 ´ 10^-5 Pa m³ mol^-1 (calc.)  S.g./density 0.7-0.8  Solubility In water 311 (pH 1), 20 031 (pH 5), 23 180 (pH 7), 34 196 (pH 9) (all in mg/l, 25 °C). In ethanol 1250, diethyl ether 243, heptane 1.1, toluene 6.7, xylene 5.8 (all in g/kg, 20 ºC); in octanol 120 g/l (25 °C). Insoluble in petroleum oils. Mono-n-butylamine salt: In water 18 g/l (30 ºC).  Stability 2,4-D is a strong acid, and forms water-soluble salts with alkali metals and amines. Hard water leads to precipitation of the calcium and magnesium salts, but a sequestering agent is included in formulations to prevent this. Photolytic DT₅₀ (simulated sunlight) 7.5 d.  pKa 2.73 

Oral Acute oral LD₅₀ for rats 639-764, mice 138 mg/kg.  Skin and eye Acute percutaneous LD₅₀ for rats >1600, rabbits >2400 mg/kg. Skin and eye irritant (rabbits). Not a skin sensitiser (guinea pigs).  Inhalation LC₅₀ (24 h) for rats >1.79 mg/l.  NOEL (2 y) for rats and mice 5 mg/kg b.w.; (1 y) for dogs 1 mg/kg b.w.  ADI (JMPR) 0.01 mg/kg b.w. [1997] (environmental assessment); 0.01 mg/kg b.w. [1996] (sum of 2,4-D and its salts and esters, as 2,4-D).  Water GV 30 mg/l (based on ADI).  Toxicity class WHO (a.i.) II; EPA (formulation) II  EC hazard Xn; R22| Xi; R36/37/38: (for salts and esters, Xn; R20/21/22) 

ECOTOXICOLOGY

2,4-D

Birds Acute oral LD₅₀ for wild ducks >1000, Japanese quail 668, pigeons 668, pheasants 472 mg/kg. LC₅₀ (96 h) for mallard ducks >5620 mg/l.  Fish Some formulations (e.g. esters) are toxic to fish, whilst others are not. LC₅₀ (96 h) for rainbow trout >100 mg/l.  Daphnia LC₅₀ (21 d) 235 mg/l.  Algae EC₅₀ (5 d) for Selenastrum capricornutum 33.2 mg/l.  Other aquatic spp. EC₅₀ (14 d) for Lemna gibba 0.58 mg/l.  Bees Not toxic to bees; LD₅₀ (oral) 104.5 mg/bee.  Worms LC₅₀ (7 d) 860 mg/kg; NOEC (14 d) 100 g/kg.  Other beneficial spp. Harmless to Trichogrammia cacoeciae, Poecilus cupreus, Aleochara bilineata. 

ENVIRONMENTAL FATE

2,4-D

EHC 29 (WHO, 1984), 84 (WHO, 1989).  EHC 84 concludes that, when used as recommended, 2,4-D does not appear to produce direct toxic effects on any animal species.  Animals In rats, following oral administration, elimination is rapid, and mainly as the unchanged substance. Following single doses of up to 10 mg/kg, excretion is almost complete after 24 hours, although, with higher doses, complete elimination takes longer. The maximum concentration in organs is reached after c. 12 hours.  Plants In plants, metabolism involves hydroxylation, decarboxylation, cleavage of the acid side-chain, and ring opening.  Soil/Environment In soil, microbial degradation involves hydroxylation, decarboxylation, cleavage of the acid side-chain, and ring opening. Half-life in soil <7 d. K_oc c. 60. For a reiew of environmental aspects of 2,4-D, see Environmental Health Criteria 84 (WHO, 1989). Rapid degradation in the soil prevents significant downward movement under normal conditions.