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2,4-Dichlorophenoxyacetic acid

2,4-Dichlorophenoxyacetic acid

Overview
2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

/herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

 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 a synthetic auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

 (plant hormone), and as such it is often used in laboratories for plant research and as a supplement in plant cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...

 media such as MS medium. It was a major ingredient in Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...

.

2,4-D was developed during World War II by a British team at Rothamsted Experimental Station
Rothamsted Experimental Station
The Rothamsted Experimental Station, one of the oldest agricultural research institutions in the world, is located at Harpenden in Hertfordshire, England. It is now known as Rothamsted Research...

, under the leadership of Judah Hirsch Quastel, aiming to increase crop yields for a nation at war.
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Encyclopedia
2,4-Dichlorophenoxyacetic acid (2,4-D) is a common systemic pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...

/herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...

 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 a synthetic auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

 (plant hormone), and as such it is often used in laboratories for plant research and as a supplement in plant cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...

 media such as MS medium. It was a major ingredient in Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...

.

History


2,4-D was developed during World War II by a British team at Rothamsted Experimental Station
Rothamsted Experimental Station
The Rothamsted Experimental Station, one of the oldest agricultural research institutions in the world, is located at Harpenden in Hertfordshire, England. It is now known as Rothamsted Research...

, 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
Wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

, maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 (corn), rice
Rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

, and similar cereal
Cereal
Cereals are grasses cultivated for the edible components of their grain , composed of the endosperm, germ, and bran...

 grass crops, because it only kills dicots (broadleaf plants), leaving behind monocots (grasses).

Genetically modified crops


Dow
Dow Chemical Company
The Dow Chemical Company is a multinational corporation headquartered in Midland, Michigan, United States. As of 2007, it is the second largest chemical manufacturer in the world by revenue and as of February 2009, the third-largest chemical company in the world by market capitalization .Dow...

 has demonstrated maize
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...

 and soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...

 resistance to 2,4-D due to insertion of a bacterial aryloxyalkanoate dioxygenase gene. This is intended as an alternative to Roundup Ready crops due to the increasing prevalence of glyphosate resistant weeds.

Mechanism of action


2,4-D is a synthetic auxin
Auxin
Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins have a cardinal role in coordination of many growth and behavioral processes in the plant's life cycle and are essential for plant body development. Auxins and their role in plant growth were first described by...

, which is a class of plant hormone
Plant hormone
Plant hormones are chemicals that regulate plant growth, which, in the UK, are termed 'plant growth substances'. Plant hormones are signal molecules produced within the plant, and occur in extremely low concentrations. Hormones regulate cellular processes in targeted cells locally and, when moved...

s. It is absorbed through the leaves and is translocated to the meristem
Meristem
A meristem is the tissue in most plants consisting of undifferentiated cells , found in zones of the plant where growth can take place....

s 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
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 salt, but more potent ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 versions exist as well.

Manufacture


2,4-D is a member of the phenoxy
Phenoxy herbicide
A phenoxy herbicide is any member of a family of chemicals related to the growth hormone indoleacetic acid . When sprayed on broad-leaf plants they induce rapid, uncontrolled growth, eventually killing them. When sprayed on crops such as wheat or corn, it selectively kills just the broad-leaf...

 family of herbicides, which include:
  • 2,4,5-Trichlorophenoxyacetic acid
    2,4,5-Trichlorophenoxyacetic acid
    2,4,5-Trichlorophenoxyacetic acid , a synthetic auxin, is a chlorophenoxy acetic acid herbicide used to defoliate broad-leafed plants. It was developed in the late 1940s and was widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns....

     (2,4,5-T)
  • 2-Methyl-4-chlorophenoxyacetic acid (MCPA)
  • 2-(2-Methyl-4-chlorophenoxy)propionic acids (mecoprop
    Mecoprop
    Mecoprop, or methylchlorophenoxypropionic acid , is a common general use herbicide found in many household weed killers and "weed-and-feed" type lawn fertilizers. It is primarily used to control broadleaf weeds...

    , MCPP)
  • 2-(2,4-Dichlorophenoxy)propionic acid (dichlorprop
    Dichlorprop
    Dichlorprop is a chlorophenoxy herbicide similar in structure to 2,4-D that is used to kill annual and perennial broadleaf weeds. It is a component of many common weedkillers...

    , 2,4-DP)
  • (2,4-Dichlorophenoxy)butyric acid (2,4-DB
    2,4-DB
    2,4-DB or 4-butyric acid is a selective systemic phenoxy herbicide used to control many annual and perennial broadleaf weeds in alfalfa, peanuts, soybeans, and other crops. Its active metabolite, 2,4-D, inhibits growth at the tips of stems and roots. It is classified in toxicity class III...

    )


2,4-D is manufactured from chloroacetic acid
Chloroacetic acid
Chloroacetic acid, industrially known as monochloroacetic acid is the organochlorine compound with the formula ClCH2CO2H. This carboxylic acid is a useful building-block in organic synthesis.-Production:...

 and 2,4-dichlorophenol
2,4-Dichlorophenol
2,4-Dichlorophenol is a chlorinated derivative of phenol with the molecular formula C6H4Cl2O. 2,4-DCP is used primarily as an intermediate in the preparation of the herbicide 2,4-dichlorophenoxyacetic acid . Annual worldwide production is estimated at 88 million pounds...

, which is itself produced by chlorination
Chlorination
Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water...

 of phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...

. Alternatively, it may be produced by the chlorination of phenoxyacetic acid. The production processes creates several contaminants including isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s, monochlorophenol, and other polychlorophenols and their acids.

Dioxin impurities


Some preparations of 2,4-D are contaminated with dioxins
Dioxins and dioxin-like compounds
Dioxins and dioxin-like compounds are by-products of various industrial processes, and are commonly regarded as highly toxic compounds that are environmental pollutants and persistent organic pollutants . They include:...

 due to the manufacturing process. Tetrachlorodibenzo-p-dioxin (TCDD) is classified as "carcinogenic to humans" by International Agency for Research on Cancer
International Agency for Research on Cancer
The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....

.

Contamination is predominantly of the type 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
2,3,7,8-Tetrachlorodibenzodioxin
2,3,7,8-Tetrachlorodibenzo-p-dioxin is a polychlorinated dibenzo-p-dioxin . It is the most potent compound of the series and became known as a contaminant in Agent Orange, a herbicide used in the Vietnam War, as well as the Seveso disaster...

 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
2,4-Dichlorophenol
2,4-Dichlorophenol is a chlorinated derivative of phenol with the molecular formula C6H4Cl2O. 2,4-DCP is used primarily as an intermediate in the preparation of the herbicide 2,4-dichlorophenoxyacetic acid . Annual worldwide production is estimated at 88 million pounds...

, 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 in exposed golfers

The defoliant and herbicide Agent Orange
Agent Orange
Agent Orange is the code name for one of the herbicides and defoliants used by the U.S. military as part of its herbicidal warfare program, Operation Ranch Hand, during the Vietnam War from 1961 to 1971. Vietnam estimates 400,000 people were killed or maimed, and 500,000 children born with birth...

, used extensively throughout the Vietnam War
Vietnam War
The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

, 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.

Applications


2,4-D is primarily used as an herbicide. It 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
Mecoprop
Mecoprop, or methylchlorophenoxypropionic acid , is a common general use herbicide found in many household weed killers and "weed-and-feed" type lawn fertilizers. It is primarily used to control broadleaf weeds...

 and dicamba
Dicamba
Dicamba is an herbicide. Brand names for formulations of this herbicide include Banvel, Oracle and Vanquish...

. Over 1,500 herbicide products contain 2,4-D as an active ingredient.

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, alternatives such as corn gluten meal
Corn gluten meal
Corn gluten meal is a byproduct of corn processing that has historically been used as an animal feed. It can also be used as an organic herbicide.- Herbicide :...

 and vinegar
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

-based products are increasingly being used to combat weeds.

Cancer risk


Different organizations have taken different stances on cancer risk of 2,4-D. On August 8, 2007, the United States Environmental Protection Agency issued a ruling that stated that existing data does not support a conclusion that links human cancer to 2,4-D exposure. The International Agency for Research on Cancer (IARC) has classified 2,4-D among the phenoxy acid herbicides MCPA
MCPA
MCPA or 2-methyl-4-chlorophenoxyacetic acid is a powerful, selective, widely-used phenoxy herbicide. The pure compound is a brown-colored powder.-History:...

 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
Nebraska
Nebraska is a state on the Great Plains of the Midwestern United States. The state's capital is Lincoln and its largest city is Omaha, on the Missouri River....

, 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
Dow Chemical Company
The Dow Chemical Company is a multinational corporation headquartered in Midland, Michigan, United States. As of 2007, it is the second largest chemical manufacturer in the world by revenue and as of February 2009, the third-largest chemical company in the world by market capitalization .Dow...

 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
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...

.

Other


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 associated with the risk of amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...

.

The amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

 salt formulations can cause eye damage (blindness
Blindness
Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...

) on contact; ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

 formulations are considered non-irritating to the eyes.

One study found that occupational exposure to 2,4-D caused male reproductive problems, including dead and malformed sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

.

Concerns regarding neurotoxicity
Neurotoxicity
Neurotoxicity occurs when the exposure to natural or artificial toxic substances, which are called neurotoxins, alters the normal activity of the nervous system in such a way as to cause damage to nervous tissue. This can eventually disrupt or even kill neurons, key cells that transmit and process...

 have been voiced with increased sensitivity to amphetamine
Amphetamine
Amphetamine or amfetamine is a psychostimulant drug of the phenethylamine class which produces increased wakefulness and focus in association with decreased fatigue and appetite.Brand names of medications that contain, or metabolize into, amphetamine include Adderall, Dexedrine, Dextrostat,...

 and thus concerns of increased risk of drug addiction among those exposed.

Treatment


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

Environmental behavior


Owing to the longevity and extent of use, 2,4-D is among the most thoroughly studied herbicides with respect to environmental properties. 2,4-D applied at 1.16 lb/acre to bluegrass turf in a laboratory experiment had a half-life of ten days. Other studies found half-life figures between 1.5 and 16 days. Soil microbes are primarily responsible for its disappearance in soil. Studies in Alaska and Canada failed to detect leaching in 22 weeks or from spring to fall, but 2,4-D has been included on the EPA list of compounds that are likely to leach from soil.

In aquatic environments microorganisms readily degrade 2,4-D and breakdown by sunlight is not a major reason for loss. Rates of breakdown increase with increased nutrients, sediment load and dissolved organic carbon. Under oxygenated conditions the half-life can be short, in the order of one week to several weeks. 2,4-D interferes with normal plant growth processes. Uptake of the compound is through leaves, stems and roots; however, it is, in general, nonpersistent. In one study when 2,4-D was applied to grass, there were 80 ppm at day zero, 45 ppm at 14 days, and 6 ppm at 56 days. Breakdown in plants is by a variety of biological and chemical pathways.

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
Catabolism
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

 have been identified for several organisms. As a result of the extensive metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...

 on environmental behavior, physiology and genetics, 2,4-D was the first herbicide for which the bacteria actively responsible for in situ
In situ
In situ is a Latin phrase which translated literally as 'In position'. It is used in many different contexts.-Aerospace:In the aerospace industry, equipment on board aircraft must be tested in situ, or in place, to confirm everything functions properly as a system. Individually, each piece may...

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. 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.

Despite its short half-life in soil and in aquatic environments, the compound has been detected in groundwater supplies in at least five States and in Canada. It has also been detected in surface waters throughout the United States at very low concentrations.

Legal issues


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.

External links


Government and academic references:

Industry website:

Health and environmental references: