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This article is about the herbicide. For the toxin found in lychee seeds, see methylene cyclopropyl acetic acid
MCPA (2-methyl-4-chlorophenoxyacetic acid) is a widely used phenoxy herbicide introduced in . It selectively controls broad-leaf weeds in pasture and cereal crops. The mode of action of MCPA is as an auxin, which are growth hormones that naturally exist in plants.[2][3]
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In investigations began at ICIs Jealott's Hill research center into the effects of auxins on plant growth looking specifically for a way to kill weeds without harming crops such as wheat and oats. William Templeman found that when indole-3-acetic acid (IAA), the naturally occurring auxin, was used at high concentrations, it could stop plant growth. In , he published his finding that IAA killed broadleaf plants within a cereal field.[4][5] Templeman and the ICI group were searching for compounds with similar or greater selective activity than IAA or 1-naphthaleneacetic acid in inhibiting the growth of weeds while not adversely affecting the growth of cereal crops. They synthesized MCPA from the corresponding phenol by exposing it to chloroacetic acid and dilute base in a straightforward substitution reaction:[6]
By the end of it was clear to the Templeman group that MCPA was one of the most active compounds tested but other auxin herbicides including 2,4-D were also effective. This work took place during World War II and was a case of multiple discovery. Four groups worked independently in the United Kingdom and the United States: the ICI team; Philip S. Nutman and associates at Rothamsted Research in the UK; Franklin D. Jones and associates at the American Chemical Paint Company; and Ezra Kraus, John W. Mitchell, and associates at the University of Chicago and the United States Department of Agriculture. All four groups were subject to wartime secrecy laws and did not follow the usual procedures of publication and patent disclosure, although ICI did file an application relating to both MCPA and 2,4-D on 7 April in the UK. In December , following a meeting at the Ministry of Agriculture the Rothamsted and ICI workers pooled resources and Nutman moved to Jealott's Hill to join the ICI effort.[5] The first publications about this group of herbicides were by other workers who were not the original inventors: the precise sequence of discovery events has been discussed.[7] MCPA was first reported in the open scientific literature by Slade, Templeman and Sexton in .[8] ICI's decision to commercialize MCPA (rather than 2,4-D, for example) was influenced by the fact that ICI had access to 2-methyl-4-chlorophenol and following extensive field trials the material was first made available to UK farmers in , as a 1% dust.[5]
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MCPA acts by mimicking the action of the plant growth hormone auxin, which results in uncontrolled growth and eventually death in susceptible plants, mainly dicotyledons.[3] 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.
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US Geological Survey estimate of MCPA use in the USA, toMCPA is used as an herbicide, generally as its salt or esterified forms. Used thus, it controls broadleaf weeds, including thistle and dock, in cereal crops and pasture. It is selective for plants with broad leaves, and this includes most deciduous trees. Clovers are tolerant at moderate application levels. It is currently classified as a restricted use pesticide in the United States: its use is mapped by the US Geological Survey, whose data show consistent use from , with a small recent decline in the ten years to , the latest date for which figures are available. The compound is now used almost exclusively in wheat.[9]
Its toxicity and biodegradation are topics of current research. One formulation is described by its manufacturer as "designed for specific markets that require the safest possible phenoxy product, primarily for use in the Pacific Northwest".[10] Though not extremely toxic,[11] it has been determined that MCPA can form complexes with metal ions and thereby increase their bioavailability,[12] and there is also work being done to utilize this ability.[13]
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Because it is inexpensive, MCPA is used in various chemical applications. Its carboxylic acid group allows the formation of conjugated complexes with metals (see above). The acid functionality makes MCPA a versatile synthetic intermediate for more complex derivatives.[14]
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The following commercial products contain MCPA:[11]
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Since MCPA is extensively used in the USA, the extensively dispersed MCPA and its biological and photochemical metabolites might be deemable as environmentally hazardous. However, current studies show that there is no resistance of MCPA to degrade in soil.
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MCPA herbicide is usually sprayed to the soil surface and plant leaves in its water solution, sometimes with additional surfactant. MCPA in soil can be absorbed by plant roots, and translocated in phloem to leaves and stems. The MCPA residue left in soil typically has a half-life of 24 days.[15] However, the degradation rate depends on environmental conditions, such as temperature and soil moisture.[16] MCPA is rather mobile in soil, and not strongly adsorbed to soil particles, with Kf = 0.94 and 1/n = 0.68 of Freundlich adsorption.[15][16]
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Wide usage of MCPA as an herbicide raises concern of environmental risks, so considerable research has been done in recent decades to evaluate the environmental risk of MCPA. MCPA can be moderately toxic to mammal and aquatic organisms, and relatively less toxic to birds.[17] MCP (4-chloro-2-methylphenol) is the intermediate in the synthesis of phenoxy herbicides, and is also the metabolite of MCPA degradation. It has been estimated that a total of tons of MCP were produced in in the EU.[18] MCP is considered very toxic to aquatic organisms. However, the concentration of MCPA and MCP detected in water and soil are lower than the predicted no-effect levels of all environmental compartments, and considered to present low potential risk.[18][19]
The carboxyl group of MCPA can form conjugated complex with metals as a ligand.[20] In the general pH range of aqueous environments, the MCPA-metal complex has higher solubility than metal ions. MCPA may be environmentally hazardous by affecting the mobility and bio-availability of heavy metals such as cadmium and lead. The acid functionality makes MCPA a versatile synthetic intermediate for more complex derivatives[21]
-COOH + M+ &#; -COOM + H+
Bio-degradation of MCPA in soilsThe MCPA can be degraded biologically in soils by plants and microorganisms. The major metabolite of MCPA degradation is MCP (4-chloro-2-methylphenol). The pathway could be the cleavage of the ether linkage, yielding MCP and acetate acid. Another pathway could be the hydroxylation of the methyl group, yielding cloxyfonac (4-Chloro-2-hydroxymethylphenoxyacetic acid). Recent studies have demonstrated that biological degradation of MCPA is enzymatically catalyzed by an α-ketoglutarate-dependent dioxygenase encoded by the tfdA gene of soil microorganisms. Soil indigenous bacteria that carry the tfdA gene could use MCPA as the sole source of carbon.[22][23]
Oxidation of MCPA by hydroxyl radicals Oxidation of MCPA by positive holes h+MCPA also could be photochemically degraded. Two scheme pathways can be proposed for the formation of the main intermediate, MCP. One scheme is MCPA oxidation by hydroxyl radical, &#;OH. The hydroxyl radical adds on the ring, followed by radical transfer to the ether carbon. With oxygen present, the addition of the hydroxyl radical leads the cleavage of the ether link, yielding MCP. The other scheme is MCPA oxidation by positive electron holes h+. The positive holes h+ polarize carboxyl group, CH2-COOH bond is split to produce 4-chloro-2-methylphenylformate. With the presence of oxygen, the positive holes h+ oxidation finally yields MCP as well.[21]
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The following alerts are based on the data in the tables below. An absence of an alert does not imply the substance has no implications for human health, biodiversity or the environment but just that we do not have the data to form a judgement.
Environmental fate Ecotoxicity Human health    Human healthModerate alert:Genotoxic; Reproduction/development effects
GENERAL INFORMATION Description A herbicide for control of annual and perennial weeds in cereals and other crops. Can also be a pesticide transformation product. Example pests controlled Annual and perennial broad-leaved weeds including Charlock, Wild radish, Dandylion, Capeweed, Fat hen and Hedge mustards Example applications Wheat; Oats; Tritical; Rye; Establish grass; Lindseed; Asparagus Efficacy & activity - Availability status Current Introduction & key dates circa , introduced UK regulatory status UK COPR regulatory status Approved Date COPR inclusion expires 31/10/ UK LERAP status Check label - may vary with formulation EC Regulation / (repealing 91/414) EC Regulation / status Approved Dossier rapporteur/co-rapporteur Poland/Netherlands Date EC / inclusion expires 15/08/ EU Candidate for substitution (CfS) No Listed in EU database Yes - as the acid Approved for use (&#;) under EC / in the following EU Member StatesAT
Austria
BE
Belgium
BG
Bulgaria
CY
Cyprus
CZ
Czech Republic
DE
Germany
DK
Denmark
EE
Estonia
EL
Greece
&#; &#; &#; &#; &#; &#; &#; &#; &#;ES
Spain
FI
Finland
FR
France
HR
Croatia
HU
Hungary
IE
Ireland
IT
Italy
huimeng supply professional and honest service.
LT
Lithuania
LU
Luxembourg
&#; &#; &#; &#; &#; &#; &#; &#; &#;LV
Latvia
MT
Malta
NL
Netherlands
PL
Poland
PT
Portugal
RO
Romania
SE
Sweden
SI
Slovenia
SK
Slovakia
&#; &#; &#; &#; &#; &#; &#; &#; &#; Approved for use (&#;) under EC / by Mutual Recognition of Authorisation and/or national regulations in the following EEA countriesIS
Iceland
NO
Norway
                  Additional information Also used in Australia, USA, Egypt Chemical structure Isomerism None Chemical formula C&#;H&#;ClNaO&#; Canonical SMILES CC1=C(C=CC(=C1)Cl)OCC(=O)[O-].[Na+] Isomeric SMILES - International Chemical Identifier key (InChIKey) STAPBGVGYWCRTF-UHFFFAOYSA-M International Chemical Identifier (InChI) InChI=1S/C9H9ClO3.Na/c1-6-4-7(10)2-3-8(6)13-5-9(11)12;/h2-4H,5H2,1H3,(H,11,12);/q;+1/p-1 2D structure diagram/image available? Yes Cambridge Crystallographic Data Centre diagrams Common Name Relationship Link MCPA Parent General status Pesticide type Herbicide Substance groups Aryloxyalkanoic acid herbicide; Phenoxyacetic herbicide Minimum active substance purity - Known relevant impurities - Substance origin Synthetic Mode of action Selective, systemic with translocation. Synthetic auxin. CAS RN -48-3 EC number 222-895-9 CIPAC number 2 US EPA chemical code - PubChem CID CLP index number 607-051-00-3 Molecular mass 222.60 PIN (Preferred Identification Name) sodium (4-chloro-2-methylphenoxy)acetate IUPAC name sodium [(4-chloro-o-tolyl)oxy]acetate CAS name sodium 2-(4-chloro-2-methylphenoxy)acetate Other status information Potential groundwater contaminant Relevant Environmental Water Quality Standards UK Environment Agency non-statutory standard for the protection of freshwater aquatic life pH <7: 12 µg l&#;¹, pH >7 80 µg l&#;¹ as annual average; pH <7: 120 µg l&#;¹, pH >7 800 µg l&#;¹ as max acceptable conc. Data as the acid. For salt water 80 µg l&#;¹ as annual average, 800 µg l&#;¹ as max acceptable conc. Non-statutory WHO drinking water guideline 0.002 mg l&#;¹ Herbicide Resistance Class (HRAC MoA class) O Herbicide Resistance Class (WSSA MoA class) 4 Insecticide Resistance Class (IRAC MoA class) Not applicable Fungicide Resistance Class (FRAC MOA class) Not applicable Examples of recorded resistance Raphanus raphanistrum, Sisymbrium orientale Physical state - Related substances & organismsA = EU regulatory and evaluation data as published by EC, EFSA (RAR, DAR & Conclusion dossiers), EMA (e.g. EU Annex III PIC DGD) ( EU - Pesticides database 5 = Verified data used for regulatory purposes
A5
- Data type Regulatory data - observed in metabolism and farm animal feeding studies EFSA Scientific Publications )A = EU regulatory and evaluation data as published by EC, EFSA (RAR, DAR & Conclusion dossiers), EMA (e.g. EU Annex III PIC DGD) ( EU - Pesticides database 5 = Verified data used for regulatory purposes
A5
- Density (g ml&#;¹) - - - Dissociation constant pKa) at 25 °C - - - - Vapour pressure at 20 °C (mPa) - - - Henry's law constant at 25 °C (Pa m³ mol&#;¹) - - - Volatilisation as max % of applied dose lost From plant surface - - - From soil surface - - - Maximum UV-vis absorption L mol&#;¹ cm&#;¹ - - - Surface tension (mN m&#;¹) - - - Degradation Property Value Source; quality score; and other information Interpretation General biodegradability - Soil degradation (days) (aerobic) DT&#;&#; (typical) - - - DT&#;&#; (lab at 20 °C) - - - DT&#;&#; (field) - - - DT&#;&#; (lab at 20 °C) - - - DT&#;&#; (field) - - - DT&#;&#; modelling endpoint - - - Note - Dissipation rate RL&#;&#; (days) on plant matrix Value - - - Note - Dissipation rate RL&#;&#; (days) on and in plant matrix Value - - - Note - Aqueous photolysis DT&#;&#; (days) at pH 7 Value - - - Note - Aqueous hydrolysis DT&#;&#; (days) at 20 °C and pH 7 Value - - - Note - Water-sediment DT&#;&#; (days) - - - Water phase only DT&#;&#; (days) - - - Air degradation As this parameter is not normally measured directly, a surrogate measure is used: &#;Photochemical oxidative DT&#;&#;&#;. Where data is available, this can be found in the Fate Indices section below. Decay in stored produce DT&#;&#; - Soil adsorption and mobility Property Value Source; quality score; and other information Interpretation Linear Kd (mL g&#;¹) - - - Koc (mL g&#;¹) - Notes and range - Freundlich Kf (mL g&#;¹) - - - Kfoc (mL g&#;¹) - 1/n - Notes and range - pH sensitivity - Fate indices Property Value Source; quality score; and other information Interpretation GUS leaching potential index - - - SCI-GROW groundwater index (μg l&#;¹) for a 1 kg ha&#;¹ or 1 l ha&#;¹ application rate Value Cannot be calculated - - Note - Potential for particle bound transport index - - - Potential for loss via drain flow - - - Photochemical oxidative DT&#;&#; (hrs) as indicator of long-range air transport risk - - - Bio-concentration factor BCF (l kg&#;¹) - - - CT&#;&#; (days) - - Known soil metabolites Metabolite Major/Minor fraction Estimated maximum occurrence fraction Notes 4-chloro 2-methylphenol Minor fraction - - cloxyfonac - - - Known groundwater metabolitesNone
Other known metabolitesNone
ECOTOXICOLOGY Terrestrial ecotoxicology Property Value Source; quality score; and other information Interpretation Mammals - Acute oral LD&#;&#; (mg kg&#;¹) - - - Mammals - Short term dietary NOEL (mg kg&#;¹) - - - (ppm diet) - - Mammals - Chronic 21d NOAEL (mg kg&#;¹ bw d&#;¹) - - - Birds - Acute LD&#;&#; (mg kg&#;¹) - - - Birds - Short term dietary (LC&#;&#;/LD&#;&#;) - - - Birds - Chronic 21d NOEL (mg kg&#;¹ bw d&#;¹) - - - Earthworms - Acute 14 day LC&#;&#; (mg kg&#;¹) - - - Earthworms - Chronic NOEC, reproduction (mg kg&#;¹) - - - Soil micro-organisms - - - Collembola Acute LC&#;&#; (mg kg&#;¹) - - - Chronic NOEC (mg kg&#;¹) - - - Non-target plants - - - - - - Honeybees (Apis spp.) Contact acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - Oral acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - Unknown mode acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - Chronic - - - Bumblebees (Bombus spp.) Contact acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - - Oral acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - - Mason bees (Osmia spp.) Contact acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - Oral acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg bee&#;¹) - - - Other bee species (1) Acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg insect&#;¹) - - - Mode of exposure - Other bee species (2) Acute LD&#;&#; (worst case from 24, 48 and 72 hour values - μg insect&#;¹) - - - Mode of exposure - Beneficial insects (Ladybirds) - - - Beneficial insects (Lacewings) - - - Beneficial insects (Parasitic wasps) - - - Beneficial insects (Predatory mites) - - - Beneficial insects (Ground beetles) - - - Aquatic ecotoxicology Property Value Source; quality score; and other information Interpretation Temperate Freshwater Fish - Acute 96 hour LC&#;&#; (mg l&#;¹) - - - Temperate Freshwater Fish - Chronic 21 day NOEC (mg l&#;¹) - - - Tropical Freshwater Fish - Acute 96 hour LC&#;&#; (mg l&#;¹) - - - Temperate Freshwater Aquatic invertebrates - Acute 48 hour EC&#;&#; (mg l&#;¹) - - - Temperate Freshwater Aquatic invertebrates - Chronic 21 day NOEC (mg l&#;¹) - - - Tropical Freshwater Aquatic invertebrates - Acute 48 hour EC&#;&#; (mg l&#;¹) - - - Aquatic crustaceans - Acute 96 hour LC&#;&#; (mg l&#;¹) - - - Sediment dwelling organisms - Acute 96 hour LC&#;&#; (mg l&#;¹) - - - Sediment dwelling organisms - Chronic 28 day NOEC, static, water (mg l&#;¹) - - - Sediment dwelling organisms - Chronic 28 day NOEC, sediment (mg kg&#;¹) - - - Aquatic plants - Acute 7 day EC&#;&#;, biomass (mg l&#;¹) - - - Algae - Acute 72 hour EC&#;&#;, growth (mg l&#;¹) - - - Algae - Chronic 96 hour NOEC, growth (mg l&#;¹) - - - Mesocosm study data NOEAEC mg l&#;¹ - - - NOEAEC mg l&#;¹ - - - HUMAN HEALTH AND PROTECTION General Property Value Source; quality score; and other information Interpretation Threshold of Toxicological Concern (Cramer Class) High (class III) - - Mammals - Acute oral LD&#;&#; (mg kg&#;¹) - - - Mammals - Dermal LD&#;&#; (mg kg&#;¹ body weight) - - - Mammals - Inhalation LC&#;&#; (mg l&#;¹) - - - Other Mammal toxicity endpoints - - - ADI - Acceptable Daily Intake (mg kg&#;¹ bw day&#;¹) 0.05 EFSA Scientific Publications )A = EU regulatory and evaluation data as published by EC, EFSA (RAR, DAR & Conclusion dossiers), EMA (e.g. EU Annex III PIC DGD) ( EU - Pesticides database 5 = Verified data used for regulatory purposes
A5
- ARfD - Acute Reference Dose (mg kg&#;¹ bw day&#;¹) 0.15 EFSA Scientific Publications )A = EU regulatory and evaluation data as published by EC, EFSA (RAR, DAR & Conclusion dossiers), EMA (e.g. EU Annex III PIC DGD) ( EU - Pesticides database 5 = Verified data used for regulatory purposes
A5
- AAOEL - Acute Acceptable Operator Exposure Level (mg kg&#;¹ bw day&#;¹) - - - AOEL - Acceptable Operator Exposure Level - Systemic (mg kg&#;¹ bw day&#;¹) 0.04 EFSA Scientific Publications )A = EU regulatory and evaluation data as published by EC, EFSA (RAR, DAR & Conclusion dossiers), EMA (e.g. EU Annex III PIC DGD) ( EU - Pesticides database 5 = Verified data used for regulatory purposes
A5
- Dermal penetration studies (%) - - - Dangerous Substances Directive 76/464 List II - - Exposure Routes Public May be absorbed through the skin Occupational Occupational exposure may occur through inhalation and dermal contact MRLs European EU MRL pesticide database  Great Britain GB MRL Register  Notes - Drinking Water Standards - - - Drinking Water MAC (μg l&#;¹) - - - Mammalian dose elimination route and rate - - - Health issues Specific human health issues Carcinogen Genotoxic Endocrine disruptorX
No, known not to cause a problem
A3
A = Chromosome aberration (EFSA database)
3 = Negative
B0
B = DNA damage/repair (EFSA database)
0 = No data
C0
C = Gene mutation (EFSA database)
0 = No data
D0
D = Genome mutation (EFSA database)
0 = No data
E2
E = Unspecified genotoxicity type (miscellaneous data source)
2 = Mixed/ambiguous results
X
No, known not to cause a problem
Reproduction / development effects Acetyl cholinesterase inhibitor Neurotoxicant?
Possibly, status not identified
X
No, known not to cause a problem
X
No, known not to cause a problem
Respiratory tract irritant Skin irritant Skin sensitiser?
Possibly, status not identified
?
Possibly, status not identified
No data found Eye irritant Phototoxicant  &#;
Yes, known to cause a problem
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No data found   General human health issues Highly toxic