|Molar mass||221.04 g/mol|
|Appearance||white to yellow powder|
|Melting point||140.5 °C (284.9 °F; 413.6 K)|
|Boiling point||160 °C (320 °F; 433 K) 0.4 mm Hg|
|Flash point||nonflammable |
|Lethal dose or concentration (LD, LC):|
LD50 (Median dose)
|500 mg/kg (oral, hamster)
100 mg/kg (oral, dog)
|US health exposure limits (NIOSH):|
|TWA 10 mg/m3|
|TWA 10 mg/m3|
IDLH (Immediate danger
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is: / ?)(|
2,4-Dichlorophenoxyacetic acid (usually referred to by its abbreviation, 2,4-D) is a common systemic herbicide used in the control of broadleaf weeds. It is a synthetic auxin (plant hormone) and one of the most widely used herbicides in the world. Since 2014 genetically modified soybeans and genetically modified maize resistant to 2,4D and glyphosate have been approved in Canada and the U.S. as well as the corresponding herbicide Enlist Duo, which contains 2,4-D choline and glyphosate. 2,4-D was one of the ingredients in Agent Orange, the herbicide widely used during the Vietnam War. According to the US National Pesticide Information Center, "the controversy regarding health effects centered around the 2,4,5-T component of the herbicide and its contaminant, dioxin."
- 1 Applications
- 2 Health effects
- 3 Usage instruction
- 4 Metabolism
- 5 Environmental behavior
- 6 Regulation
- 7 Mode of action
- 8 Manufacture
- 9 Genetically modified crops
- 10 History
- 11 See also
- 12 References
- 13 External links and further reading
2,4-D is primarily used as a synthetic auxin analog, as an herbicide. It is sold in various formulations under a wide variety of brand names. 2,4-D can be found in commercial lawn herbicide mixtures. These products often contain other active ingredients including mecoprop and dicamba. Over 1,500 herbicide products contain 2,4-D as an active ingredient.
2,4-D is commonly used for weed control in lawns and other turf and control of weeds and brush along fences and highway and railroad rights of way. In agriculture it is used as a soil application in orchards, while foliar application is used in grass hayfields, pastures, cereal grains, including corn and sorghum (occasionally).:35–36
2,4-D continues to be used, where legal, for its low cost. Where municipal lawn pesticide bylaws exist, such as in Canada, alternatives must be used.
A 2010 monitoring study conducted in the US and Canada found that "current exposures to 2,4-D are below applicable exposure guidance values."
According to the U.S. Environmental Protection Agency, "The toxicity of 2,4-D depends on its chemical forms, including salts, esters, and an acid form. 2,4-D generally has low toxicity for humans, except certain acid and salt forms can cause eye irritation. Swimming is restricted for 24 hours after application of certain 2,4-D products applied to control aquatic weeds to avoid eye irritation." As of 2005[update] the median lethal dose or LD50 determined in acute toxicity rat studies was 639 mg/kg.
On August 8, 2007, the EPA issued a ruling that existing data do not support a link between human cancer and 2,4-D exposure.
A 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions. None of the scientists thought the weight of the evidence indicated that 2,4-D was a “known” or “probable” cause of human cancer. The predominant opinion indicated that it is possible that 2,4-D can cause cancer in humans, although not all of the panelists believed the possibility was equally likely: one thought the possibility was strong, leaning toward probable, and five thought the possibility was remote, leaning toward unlikely. Two panelists believed it unlikely that 2,4-D can cause cancer in humans.
A July 2013 Four Corners investigation found elevated levels of dioxins in a generic version of 2,4-D, one of Australia's most widely used herbicides. One scientist said the product tested by Four Corners, which was imported from China, had "one of the highest dioxin readings for 2,4-D in the last 10 to 20 years, and could pose potential health risks."
When radioactively-labeled 2,4 D was fed to livestock, 90% or more of the total radioactive residue (TRR) was shed in urine unchanged or as conjugated forms of 2,4-D. A relatively small portion of 2,4 D was metabolized into dichlorophenol and into dichloroanisole and 4-chlorophenoxyacetic acid. (6.9% of the TRR in milk) and 2,4-dichlorophenol13 (5% of the TRR in milk; 7.3% of the TRR in eggs and 4% of the TRR in chicken liver). Residue levels in kidney were the highest.:21
2,4-D amine salts and esters are not persistent under most environmental conditions. The degradation of 2,4-D is rapid (half life of 6.2 days) in aerobic mineral soils.:54 2,4-D is broken down by microbes in soil, in processes that involve hydroxylation, cleavage of the acid side-chain, decarboxylation, and ring opening. The ethyl hexyl form of the compound is rapidly hydrolyzed in soil and water to form the 2,4-D acid. 2,4-D has a low binding affinity in mineral soils and sediment, and in those conditions is considered intermediately to highly mobile, and therefore likely to leach if not degraded.
In aerobic aquatic environments, the half life is 15 days, while in anaerobic aquatic environments, 2,4-D was moderately persistent to persistent (half life of = 41 to 333 days). 2,4-D has been detected in streams and shallow groundwater at low concentrations, in both rural and urban areas. Breakdown is pH dependent.
"The ester forms of 2,4-D can be highly toxic to fish and other aquatic life. 2,4-D generally has moderate toxicity to birds and mammals, is slightly toxic to fish and aquatic invertebrates, and is practically nontoxic to honeybees" per EPA.[date missing]
A number of 2,4-D-degrading bacteria have been isolated and characterized from a variety of environmental habitats. Metabolic pathways for the compound’s degradation have been available for many years, and genes encoding 2,4-D catabolism have been identified for several organisms. As a result of the extensive metadata on environmental behavior, physiology and genetics, 2,4-D was the first herbicide for which the bacteria actively responsible for in situ degradation was demonstrated. This was accomplished using the technique of DNA-based stable isotope probing, which enables a microbial function (activity), such as degrading a chemical, to be linked with the organism’s identity without the need to culture the organism involved.
Maximum residue limits were first set in the EU in 2002 and re-evaluated by the European Food Safety Authority in 2011. EFSA concluded that the codex maximum residue limits were "not expected to be of concern for European consumers".:26 The total chronic exposure represented less than 10% of the acceptable daily intake (ADI).:28 Concern over 2,4-D is such that it is currently not approved for use on lawns and gardens in Denmark, Norway, Kuwait and the Canadian provinces of Québec and Ontario. 2,4-D use is severely restricted in the country of Belize. In 2008, Dow AgroScience, LLC sued the federal government for allowing Quebec to ban 2,4-D, but settled in 2011.
In 2005, the US EPA approved the continued use of 2,4-D. On July 10, 2013 the Pest Management Regulatory Agency in Canada updated the re-evaluation notice of 2,4-D stating that the 2,4-D registrants had provided it with required data and deemed them acceptable. On April 18, 2012, EPA denied the petition filed November 6, 2008 by the Natural Resources Defense Council (NRDC) to revoke all tolerances and to cancel all registrations of 2,4-D. EPA stated that recent new study and EPA’s comprehensive review confirmed EPA’s previous finding that the 2,4-D tolerances are safe at anticipated exposure.
In October 2014, the US EPA registered Enlist Duo, a herbicide containing the less volatile 2,4-D choline salt, glyphosate, and an anti-drift agent, for use in six states: Illinois, Indiana, Iowa, Ohio, South Dakota, and Wisconsin.
On 21 August 2013 the Australian Pesticides and Veterinary Medicines Authority (APVMA) banned selected 2,4-D high volatile ester (HVE) products due to their environmental hazards. HVE 2, 4-D products had already been banned in Europe and North America for twenty years; low volatile ester products continue to be available in Australia and worldwide. In July 2013 APVMA published their report findings.
Mode of action
2,4-D is a synthetic auxin, which is a class of plant hormones. It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues, causing stem curl-over, leaf withering, and eventual plant death. 2,4-D is typically applied as an amine salt, but more potent ester versions exist as well.
2,4-D is manufactured from chloroacetic acid and 2,4-dichlorophenol, which is itself produced by chlorination of phenol. Alternatively, it may be produced by the chlorination of phenoxyacetic acid. The production processes create several contaminants including di-, tri-, and tetrachlorodibenzo-p-dioxin isomers and N-nitrosamines, as well as monochlorophenol.
Genetically modified crops
In 2010 Dow published that it had created genetically modified soybeans made resistant to 2,4-D by insertion of a bacterial aryloxyalkanoate dioxygenase gene, aad1.:1 Dow intended it to be used as an alternative or complement to Roundup Ready crops due to the increasing prevalence of glyphosate resistant weeds.
As of April 2014 genetically modified maize and soybeans resistant to 2,4-D and glyphosate have been approved in Canada. In September 2014 the USDA also approved Dow's maize and soybeans, and in October the EPA registered the "Enlist Duo" herbicide containing 2, 4-D and glyphosate.
2,4-D was first published by Robert Pokorny in 1941, who was working in the US; the related compound, MCPA, was discovered at about the same time by scientists in UK.:Sec 7.1  Both compounds were developed afterwards as part of a clandestine wartime effort to create chemical warfare agents for use in World War II; 2,4-D was not used this way in WWII.:Sec 7.1
2,4-D was brought to market as an herbicide called "Weedone" starting in 1945 by the American Chemical Paint Company. It revolutionized weed control, as it was the first compound that, at low doses, could selectively control dicots (broadleaf plants), but not most monocots - narrow leaf crops like wheat, maize (corn), rice, and similar cereal grass crops. At a time when labor was scarce and there was a huge need for increased food production, it literally "replaced the hoe".:Sec 7.1
2,4-D was one of the ingredients in Agent Orange, the herbicide widely used during the Vietnam war. According to the US National Pesticide Information Center, "the controversy regarding health effects centered around the 2,4,5-T component of the herbicide and its contaminant, dioxin."
In the 2000s Dow Agrosciences developed a new choline salt version of 2,4-D (2,4-D choline) that Dow included in its "Enlist Duo" herbicide along with glyphosate and an agent that reduces drift; the choline salt form of 2,4-D is less volatile than 2,4-D.
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|Wikimedia Commons has media related to 2,4-Dichlorophenoxyacetic acid.|
- CDC - NIOSH Pocket Guide to Chemical Hazards
- Overview of the toxic effects of 2,4-D Sierra Club Canada January, 2005
- "Review of 2,4-dichlorophenoxyacetic acid (2,4-D) biomonitoring and epidemiology" Review of the literature by Dow scientists Crit Rev Toxicol. Oct 2012