2,4-Dichlorophenoxyacetic acid

From Wikipedia, the free encyclopedia
  (Redirected from 2,4-D)
Jump to: navigation, search
2,4-Dichlorophenoxyethanoic acid
2,4-Dichlorophenoxyacetic acid
Ball-and-stick model of 2,4-dichlorophenoxyacetic acid
CAS number 94-75-7 YesY
PubChem 1486
ChemSpider 1441 YesY
UNII 2577AQ9262 N
KEGG C03664 YesY
ChEBI CHEBI:28854 YesY
Jmol-3D images Image 1
Molecular formula C8H6Cl2O3
Molar mass 221.04 g mol−1
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
Solubility in water 900 mg/L
Related compounds
Related compounds 2,4,5-T, Dichlorprop
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
Infobox references

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 one of the most widely used herbicides in the world. 2,4-D is a synthetic auxin (plant hormone), and as such it is often used in laboratories for plant research and as a supplement in plant cell culture media such as MS medium.

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."[2]


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.[3]:Sec 7.1 [4] 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.[3]:Sec 7.1

The first publication of 2,4-D's herbicidal activity came in 1944.[5][6]

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.[3] At a time when labor was scarce and there was a huge need for increased food production, it literally "replaced the hoe".[3]: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."[2]

Genetically modified crops[edit]

Dow has demonstrated soybean resistance to 2,4-D due to insertion of a bacterial aryloxyalkanoate dioxygenase gene.[7] Using a combination of the choline salt of 2,4-D and glyphosate in the Enlist Weed Control System on 2,4-D and glyphosate resistant transgenic crops, Dow intends its use as an alternative to Roundup Ready crops due to the increasing prevalence of glyphosate resistant weeds. Department of Agriculture approval is pending for a 2,4-D resistant corn.[8]

Mode of action[edit]

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.

Effect of 2-4-d foliar application.


2,4-D is a member of the phenoxy family of herbicides.

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 isomers, monochlorophenol, and other polychlorophenols and their acids.[9]

Containers of 2-4 D(ow) weed killer, ca. 1947


2,4-D is primarily used as an herbicide.[10] 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.[citation needed]

2,4-D is most commonly used for:

  • Weed control in lawns and other turf
  • No-till burndown
  • Control of weeds and brush along fences and highway and railroad rights of way
  • Conifer release (control of broad-leaf trees in conifer plantings)
  • Grass hayfields and pastures
  • Cereal grains
  • Corn and sorghum (occasionally)
  • As a synthetic auxin analog

2,4-D continues to be used, where legal, for its low cost. However, where municipal lawn pesticide bylaws exist, such as in Canada,[11] 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."[12]


According to a FAQ published by the EPA:

  • "2,4-D products can be safely used by following label directions.
    • 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.
    • 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.
      • The ester forms of 2,4-D can be highly toxic to fish and other aquatic life.
      • Carefully follow label directions to avoid harmful effects."[13]

Acute toxicity[edit]

The LD50 determined in acute toxicity rat studies is 639 mg/kg.[14]

While urinary alkalinisation has been used in acute poisonings, evidence to support its use is poor.[15]


It was assumed that because of improved manufacturing processes that there were no longer any dangerous dioxins in 2,4-D; however, a July 2013 Four Corners investigation found "elevated levels" of dangerous 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."[16]

Cancer risk[edit]

On August 8, 2007, the U.S. Environmental Protection Agency issued a ruling that stated that existing data do not support a link between human cancer and 2,4-D exposure.[17]

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.[18]

Much older data had previously used in a 1987 report by the International Agency for Research on Cancer (IARC) which concluded that some chlorphenoxy herbicides including 2,4-D, MCPA and 2,4,5-T as a group were classified as a class 2B carcinogen - "possibly carcinogenic to humans".[19]

Environmental behavior[edit]

Owing to the longevity and extent of use, 2,4-D has been evaluated many times by regulators and review committees.[20][21]

2,4-D amine salts and esters are not persistent under most environmental conditions.[2] The degradation of 2,4-D is rapid (half life of 6.2 days) in aerobic mineral soils.[14]: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] 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.[2]

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.[2]

Microbial breakdown[edit]

A number of 2,4-D-degrading bacteria have been isolated and characterized from a variety of environmental habitats.[22][23] 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.[24] 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.[25] This advancement has been particularly beneficial for the study of soil microorganisms, since only a very small fraction of the thousands of species present in highly diverse soil bacterial communities can be isolated in pure culture.

Legal issues[edit]

Minimum residue levels were re-evaluated by the European Union and the results were published in 2011.[26] 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[27] and Ontario.[28] 2,4-D use is severely restricted in the country of Belize. In 2005, the United States Environmental Protection Agency approved the continued use of 2,4-D.[14] 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 provided the PMRA with required data submissions and deemed such data acceptable.[29] 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.[30][31]

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.[32] In July 2013 APVMA published their report findings.[33]

See also[edit]


  1. ^ "ChemIndustry site". Chemindustry.com. Retrieved 2014-05-03. 
  2. ^ a b c d e f National Pesticide Information Center NPIC 2,4-D Technical Fact Sheet
  3. ^ a b c d Andrew H. Cobb, John P. H. Reade. Herbicides and Plant Physiology. Wiley-Blackwell; 2nd edition (October 25, 2010) ISBN 978-1405129350
  4. ^ Robert Pokorny New Compounds. Some Chlorophenoxyacetic Acids J. Am. Chem. Soc., 1941, 63 (6), pp 1768–1768 doi:10.1021/ja01851a601
  5. ^ J. H. Quastel. 2,4-dichlorophenoxyacetic acid (2,4-D) as a selective herbicide. Chapter 45 (pp 244-249) in Advances in Chemistry, Vol. 1: Agricultural Control Chemicals: Collected Papers from the Symposia on Economic Poisons presented before the Division of Agricultural and Food Chemistry of the American Chemical Society at the 115th national meeting in San Francisco, March 28 to April 1, 1949, and the 116th national meeting in Atlantic City, September 18 to 23, 1949. American Chemical Society, 1950. Washington, D.C. ISBN 9780841224421
  6. ^ Hamner CL, Tukey HB. The Herbicidal Action of 2,4 Dichlorophenoxyacetic and 2,4,5 Trichlorophenoxyacetic Acid on Bindweed. Science 18 August 1944: 154-155. PMID 17778584
  7. ^ Wright TR et al. Robust crop resistance to broadleaf and grass herbicides provided by aryloxyalkanoate dioxygenase transgenes. Proc Natl Acad Sci U S A. 2010 Nov 23;107(47):20240-5. doi 10.1073/pnas.1013154107 PMCID 2996712 PMID 21059954
  8. ^ Andrew Pollack (April 25, 2012). "Dow Weed Killer, Nearing Approval, Runs Into Opposition". The New York Times. Retrieved April 25, 2012. 
  9. ^ "2,4-Dichlorophenoxyacetic Acid (2,4-D)". International Programme on Chemical Safety. Retrieved 22 June 2010. 
  10. ^ Record in the Household Products Database of NLM
  11. ^ "Private Property Pesticide By-laws In Canada" (PDF). Retrieved 2014-05-03. 
  12. ^ Aylward LL et al. Biomonitoring data for 2,4-dichlorophenoxyacetic acid in the United States and Canada: interpretation in a public health risk assessment context using Biomonitoring Equivalents. Environ Health Perspect. 2010 Feb;118(2):177-81. doi: 10.1289/ehp.0900970. PMCID PMC2831914 PMID 20123603
  13. ^ "Ingredients Used in Pesticide Products 2,4-D". www2.epa.gov. EPA. Retrieved November 6, 2014. 
  14. ^ a b c US EPA 2,4-D Reregistration Eligibility Decision, 2005. Associated RED Fact sheet
  15. ^ Roberts DM, Buckley NA (2007). Roberts, Darren M, ed. "Urinary alkalinisation for acute chlorophenoxy herbicide poisoning". Cochrane Database Syst Rev (1): CD005488. doi:10.1002/14651858.CD005488.pub2. PMID 17253558. 
  16. ^ Four Corners By Janine Cohen. "Four Corners investigation finds dangerous dioxins in widely used herbicide 24D". Abc.net.au. Retrieved 2014-05-03. 
  17. ^ "EPA: Federal Register: 2,4-D, 2,4-DP, and 2,4-DB; Decision Not to Initiate Special Review". Epa.gov. Retrieved 2014-05-03. 
  18. ^ Ibrahim, MA; Bond, GG; Burke, TA; Cole, P; Dost, FN; Enterline, PE et al. (1991). "Weight of the evidence on the human carcinogenicity of 2,4-D.". Environ Health Perspect 96: 213–22. doi:10.1289/ehp.9196213. PMC 1568222. PMID 1820267. 
  19. ^ Chlorphenoxy Herbicides (Group 2B) in IARC monographs on the evaluation of carcinogenic risks to humans: An updating of IARC Monographs volumes 1 to 42. Supplement 7, WHO, Lyon, France 1987.
  20. ^ von Stackelberg K. A Systematic Review of Carcinogenic Outcomes and Potential Mechanisms from Exposure to 2,4-D and MCPA in the Environment. J Toxicol. 2013;2013:371610. PMID 23533401 PMC 3600329
  21. ^ Andrew Pollack for the New York Times. April 9, 2012 E.P.A. Denies an Environmental Group’s Request to Ban a Widely Used Weed Killer
  22. ^ Cavalca, L., A. Hartmann, N. Rouard, and G. Soulas. 1999. Diversity of tfdC genes: distribution and polymorphism among 2,4-dichlorophenoxyacetic acid degrading soil bacteria. FEMS Microbiology Ecology 29: 45-58.
  23. ^ Suwa Y., Wright A.D., Fukimori F., Nummy K.A., Hausinger R.P., Holben W.E., Forney L.J. (1996). "Characterization of a chromosomally encoded 2,4-dichlorophenoxyacetic acid alpha-ketoglutafate dioxygenase from Burkholderia sp. strain RASC". Applied and Environmental Microbiology 62: 2464–2469. 
  24. ^ Cupples A.M., Sims G.K. (2007). "Identification of In Situ 2,4-Dichlorophenoxyacetic Acid-Degrading Soil Microorganisms using DNA-Stable Isotope Probing". Soil Biology and Biochemistry 39: 232–238. doi:10.1016/j.soilbio.2006.07.011. 
  25. ^ Radajewski S., Ineson P., Parekh N.R., Murrell J.C. (2000). "Stable-isotope probing as a tool in microbial ecology". Nature 403: 646–649. doi:10.1038/35001054. 
  26. ^ European Food Safety Authority. Reasoned opinion of EFSA: Review of the existing maximum residue levels (MRLs) for 2,4-D according to Article 12 of Regulation (EC) No 396/2005 EFSA Journal 2011;9(11):2431. doi:10.2903/j.efsa.2011.2431
  27. ^ "The Pesticides Management Code - Protecting the environment and health in our green spaces". Mddep.gouv.qc.ca. 2005-04-03. Retrieved 2014-05-03. 
  28. ^ "Ministry of the Environment | Ontario.ca". Ene.gov.on.ca. Retrieved 2014-05-03. 
  29. ^ "Health Canada Pest Management Regulatory Agency Re-evaluation Update 2,4-D REV2013-02". Hc-sc.gc.ca. Retrieved 2014-05-03. 
  30. ^ "Petition to Revoke All Tolerances and Cancel All Registrations for the Pesticide 2,4-Dichlorophenoxyacetic Acid (2,4-D); Notice of Availability" (Docket Folder Summary). Docket ID: EPA-HQ-OPP-2008-0877 Agency: EPA. United States Environmental Protection Agency. April 7, 2012. Retrieved September 12, 2012. 
  31. ^ "2,4-D; Order Denying NRDC's Petition To Revoke Tolerances" (Order). Federal Register 77 (75 (Wednesday, April 18, 2012)): 23135–23158. April 18, 2012. Retrieved September 12, 2012. Petitions to Revoke Tolerances; Denials: Natural Resources Defense Council, 2,4-dichlorophenoxyacetic acid (2-4D) Document ID: EPA-HQ-OPP-2008-0877-0446 Document Type: Rule Docket ID: EPA-HQ-OPP-2008-0877 
  32. ^ "APVMA 2,4-D Review webpage". APVMA. 
  33. ^ "Annex to the APVMA's Preliminary Review Findings (Environment) Part 1 2,4-D Esters Volume 1 Review Summary April 2006". APVMA. July 2013. Retrieved July 2014. 

External links and further reading[edit]