|Molar mass||221.04 g mol−1|
|Appearance||white to yellow powder|
140.5 °C (413.5 K)
160 °C (0.4 mm Hg)
|Solubility in water||900 mg/L (25 °C)|
|Related compounds||2,4,5-T, Dichlorprop|
(what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic herbicide used in the control of broadleaf weeds. It is the most widely used herbicide in the world, and the third most commonly used in North America. 2,4-D is also an important synthetic auxin, often used in laboratories for plant research and as a supplement in plant cell culture media such as MS medium.
2,4-D was developed during World War II by a British team at Rothamsted Experimental Station, under the leadership of Judah Hirsch Quastel, aiming to increase crop yields for a nation at war. When it was commercially released in 1946, it became the first successful selective herbicide and allowed for greatly enhanced weed control in wheat, maize (corn), rice, and similar cereal grass crop, because it only kills dicots (two seed leaves), leaving behind monocots (one seed leaf).
2,4-D is a synthetic auxin, which is a class of plant growth regulators. 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 sold in various formulations under a wide variety of brand names. 2,4-D can be found in lawn herbicide mixtures such as "Weed B Gon MAX", "PAR III", "Trillion", "Tri-Kil", "Killex" and "Weedaway Premium 3-Way XP Turf Herbicide". All of these mixtures typically contain three active ingredients: 2,4-D, mecoprop and dicamba. Over 1,500 herbicide products contain 2,4-D as an active ingredient.
2,4-D is most commonly used for:
2,4-D continues to be used, where legal, for its low cost. However, where municipal lawn pesticide bylaws exist, such as in Canada, alternatives such as corn gluten meal and vinegar based products are increasingly being used to combat weeds.
The LD50 determined in an acute toxicity rat study is 639 mg/kg. Single oral doses of 5 and 30 mg/kg body weight did not cause any acute toxic effects in human volunteers. This chemical has been observed to increase the risk of Amyotrophic lateral sclerosis (ALS) (see Burns et al. Occup Environ Med. 2001 Jan;58(1):24-30).
Different organizations have taken different stances on 2,4-D's cancer risk. On August 8, 2007, the United States Environmental Protection Agency issued a ruling which stated existing data does not support a conclusion that links human cancer to 2,4-D exposure. However, the International Agency for Research on Cancer (IARC) has classified 2,4-D among the phenoxy acid herbicides MCPA and 2,4,5-T as a class 2B carcinogen - possibly carcinogenic to humans.  A 1995 panel of 13 scientists reviewing studies on the carcinogenicity of 2,4-D had divided opinions, but the predominant opinion was that it is possible that 2,4-D causes cancer in humans.
A 1990 study of farmers in Nebraska, even when adjusting for exposure to other chemicals, found that 2,4-D exposure substantially increased the risk of Non-Hodgkin's lymphoma (NHL). A 2000 study of 1517 former employees of Dow Chemical Company who had been exposed to the chemical in manufacturing or formulating 2,4-D found no significant increase in risk of mortality due to NHL following 2,4-D exposure, but did find an increase in risk of mortality due to amyotrophic lateral sclerosis. 
2,4-D is a member of the phenoxy family of herbicides, which include:
2,4-D is manufactured from chloroacetic acid and 2,4-dichlorophenol, which is itself produced by chlorination of phenol. The production process creates several contaminants including isomers, monochlorophenol, and other polychlorophenols and their acids.
The powerful defoliant and herbicide Agent Orange, used extensively throughout the Vietnam War, contained 2,4-D. The controversies associated with the use of Agent Orange were associated with a contaminant (dioxin) in the 2,4,5-T component. However, 2,4-D is still contaminated to some extent with dioxins, predominately those with 2 or 3 chlorine atoms. Another form of dioxin, 2,7-dichlorodibenzo-p-dioxin (DCDD), an inevitable by-product of 2,4-D manufacturing, was found to be "equipotent" to dioxin TCDD in its toxic effect on the immunity of mice. TCDD received all the publicity while the DCDD component was largely forgotten. To this day DCDD is not regulated or monitored by the EPA and PMRA, even though DCDD levels could be at much higher levels than TCDD. The typical smell of 2,4-D is the break-down product 2,4-dichlorophenol which is a suspected endocrine disrupter and possible carcinogen. 2,4-D is toxic to the liver at small dosages. Increases in liver function tests, jaundice, acute hepatitis, lobular and portal inflammation indicative of a toxic reaction, as well as permanent damage leading to cirrhosis has been reported in exposed golfers 
2,4-D has been evaluated by the European Union and included on its list of approved herbicides, stating inter alia that "the review [of 2,4-D] has established that the residues arising from the proposed uses, consequent on application consistent with good plant protection practice, have no harmful effects on human or animal health." Concern over 2,4-D is such that it is currently not approved for use on lawns and gardens in Sweden, Denmark, Norway, Kuwait and the Canadian provinces of Québec  and Ontario . 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. In Canada, the Pest Management Regulatory Agency (PMRA) has placed a condition of registration on 2,4-D such that the 2,4-D registrant(s) must provide the PMRA with a required developmental neurotoxicity study by September 20, 2009. According to the PMRA, the due date of the study has since been extended to early 2010.
Government and academic references:
Health and environmental references: