(2,4-dichlorophenoxy)acetic acid

(2,4-dichlorophenoxy)acetic acid

C₈H₆Cl₂O₃

Colourless powder, with a slight phenolic odour.

140.5℃

1.86 ×10^-2 mPa

1.508 (20 ℃)

In water 311 (pH 1), 20 031 (pH 5), 23 180 (pH 7), 34 196 (pH 9) (all in mg/l, 25 ℃). In ethanol 1250, diethyl ether 243, heptane 1.1, toluene 6.7, xylene 5.8 (all in g/kg, 20 ℃); in octanol 120 g/l (25 ℃). Insoluble in petroleum oils.

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.

1.32 ×10^-5 Pa m³ mol^-1 (calc.)

2.73

Synthetic auxin (acting like indolylacetic acid).   

Selective systemic herbicide. Salts are readily absorbed by the roots, whilst esters are readily absorbed by the foliage. Translocation occurs, with accumulation principally at the meristematic regions of shoots and roots. Acts as a growth inhibitor.

Post-emergence control of annual and perennial broad-leaved weeds in cereals, maize, sorghum, grassland, established turf, grass seed crops, orchards (pome fruit and stone fruit), cranberries, asparagus, sugar cane, rice, forestry, and on non-crop land (including areas adjacent to water), at 0.28-2.3 kg/ha. Control of broad-leaved aquatic weeds. The isopropyl ester can also be used as a plant growth regulator to prevent premature fruit fall in citrus fruit.

Phytotoxic to most broad-leaved crops, especially cotton, vines, tomatoes, ornamentals, fruit trees, oilseed rape and beet.

FAO/WHO 77, 79, 80, 82, 92 (see part 2 of the Bibliography). Toxicity and hazards to man, domestic animals and wildlife have been reviewed (J. M. Way, Residue Rev., 1969, 26, 37).  IARC ref. 15, 41; Suppl. 7  class chlorophenoxy herbicides classified as 2B, based on epidemiology of production. More recent evidence (M. Kogevinas et al., Am. J. Epidemiol., 145(12), 1061 (1997)) relates this to dioxin contamination of early production. Not relevant to current processes. 

Acute oral LD₅₀ for rats 639-764, mice 138 mg/kg.

Acute percutaneous LD₅₀ for rats >1600, rabbits >2400 mg/kg. Skin and eye irritant (rabbits). A skin sensitiser (guinea pigs).

LC₅₀ (24 h) for rats >1.79 mg/l.

(2 y) for rats and mice 5 mg/kg b.w.; (1 y) for dogs 1 mg/kg b.w.

(JMPR) 0.01 mg/kg b.w. [2001, 1997]; 0.01 mg/kg b.w. [1996] (sum of 2,4-D and its salts and esters, as 2,4-D); [EEC] 0.05 mg/kg b.w. (2001).

WHO (a.i.) II; EPA (formulation) II

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.

ome formulations (e.g. esters) are toxic to fish, whilst others are not. LC₅₀ (96 h) for rainbow trout >100 mg/l.

LC₅₀ (21 d) 235 mg/l.

EC₅₀ (5 d) for Selenastrum capricornutum 33.2 mg/l.

LC₅₀ (7 d) 860 mg/kg; NOEC (14 d) 100 g/kg.

In plants, metabolism involves hydroxylation, decarboxylation, cleavage of the acid side-chain, and ring opening.

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.

In soil, microbial degradation involves hydroxylation, decarboxylation, cleavage of the acid side-chain, and ring opening. Half-life in soil <7 d. Koc c. 60. For a review 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. 

analysis of 2,4-D, salts, esters and mixed combination products by acid-base titration, by glc (CIPAC Handbook, 1985, 1C, 2060, 2257; 1994, F, 292-319; Herbicides 1977, pp. 6-21), by rplc (AOAC Methods, 17th Ed., 971.07, 976.03, 978.05, 984.07; CIPAC Handbook, 1985, 1C, 2060; 1988, D, 51), by hplc (ibid., 1983, 1757), or by i.r. spectrometry (ibid., 1998, H, 131). Free phenol impurity determined by glc (CIPAC Handbook, 1994, F, 197), hplc (ibid., 1994, F, 362) or electrochemically (ibid., 1994, F, 368). Residues determined by glc of derivatives (Pestic. Anal. Man., 1979, I, 201-D; Anal. Methods Pestic. Plant Growth Regul., 1972, 6, 630) or by hplc (M. Meier et al.,Fresenius Z. Anal. Chem.,1989, 334, 235). In drinking water, by conversion to methyl ester with diazomethane, then glc with ECD (AOAC Methods, 17th Ed., 992.32).

HDPE