MCPA

Last updated
MCPA
MCPA structure.png
MCPA molecule ball.png
Names
Preferred IUPAC name
(4-Chloro-2-methylphenoxy)acetic acid
Other names
2-(4-Chloro-2-methylphenoxy)acetic acid
4-Chloro-o-tolyloxyacetic acid
MCPA
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.002.146 OOjs UI icon edit-ltr-progressive.svg
KEGG
PubChem CID
UNII
  • InChI=1S/C9H9ClO3/c1-6-4-7(10)2-3-8(6)13-5-9(11)12/h2-4H,5H2,1H3,(H,11,12) Yes check.svgY
    Key: WHKUVVPPKQRRBV-UHFFFAOYSA-N Yes check.svgY
  • InChI=1/C9H9ClO3/c1-6-4-7(10)2-3-8(6)13-5-9(11)12/h2-4H,5H2,1H3,(H,11,12)
    Key: WHKUVVPPKQRRBV-UHFFFAOYAG
  • Cl-C1=CC=C(OCC(=O)O)C(C)=C1
Properties
C9H9ClO3
Molar mass 200.62 g·mol−1
AppearanceWhite to light brown solid
Density 1.18-1.21 g/cm3
Melting point 114 to 118 °C (237 to 244 °F; 387 to 391 K)
825 mg/L (23 °C), [1]
amine salt[ which? ]: 866 g/L
ester[ which? ]: 5 mg/L
Hazards
Safety data sheet (SDS) External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

MCPA (2-methyl-4-chlorophenoxyacetic acid) is a widely used phenoxy herbicide introduced in 1945. 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]

Contents

History

In 1936 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 1940, 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]

2-methyl-4-chlorophenol + ClCH2CO2H + base → MCPA + base·HCl (hydrochloric acid)

By the end of 1941 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 1941 in the UK. In December 1942, 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 1945. [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 1946, as a 1% dust. [5]

Mode of action

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.

Commercial use

US Geological Survey estimate of MCPA use in the USA, 1992 to 2017 MCPA use in the USA in 2017.png
US Geological Survey estimate of MCPA use in the USA, 1992 to 2017

MCPA 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 1992, with a small recent decline in the ten years to 2017, 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]

Chemical use

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]

Brand names

The following commercial products contain MCPA: [11]

Degradation in soil

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.

Behaviors in soil

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]

Environmental risks

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 15000 tons of MCP were produced in 1989 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

Bio-degradation of MCPA in soils Bio-degradation of MCPA.png
Bio-degradation of MCPA in soils

The 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]

Photo-degradation

Oxidation of MCPA by hydroxyl radicals Oxidation of MCPA by hydroxyl radicals.png
Oxidation of MCPA by hydroxyl radicals
Oxidation of MCPA by positive holes h+ Oxidation of MCPA by positive holes h+.png
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]

Related Research Articles

<span class="mw-page-title-main">Herbicide</span> Type of chemical used to kill unwanted plants

Herbicides, also commonly known as weed killers, are substances used to control undesired plants, also known as weeds. Selective herbicides control specific weed species while leaving the desired crop relatively unharmed, while non-selective herbicides kill plants indiscriminately. The combined effects of herbicides, nitrogen fertilizer, and improved cultivars has increased yields of major crops by 3x to 6x from 1900 to 2000.

<span class="mw-page-title-main">Glyphosate</span> Systemic herbicide and crop desiccant

Glyphosate is a broad-spectrum systemic herbicide and crop desiccant. It is an organophosphorus compound, specifically a phosphonate, which acts by inhibiting the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP). It is used to kill weeds, especially annual broadleaf weeds and grasses that compete with crops. Its herbicidal effectiveness was discovered by Monsanto chemist John E. Franz in 1970. Monsanto brought it to market for agricultural use in 1974 under the trade name Roundup. Monsanto's last commercially relevant United States patent expired in 2000.

<span class="mw-page-title-main">Auxin</span> Plant hormone

Auxins are a class of plant hormones with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essential for plant body development. The Dutch biologist Frits Warmolt Went first described auxins and their role in plant growth in the 1920s. Kenneth V. Thimann became the first to isolate one of these phytohormones and to determine its chemical structure as indole-3-acetic acid (IAA). Went and Thimann co-authored a book on plant hormones, Phytohormones, in 1937.

<span class="mw-page-title-main">Indole-3-acetic acid</span> Chemical compound

Indole-3-acetic acid is the most common naturally occurring plant hormone of the auxin class. It is the best known of the auxins, and has been the subject of extensive studies by plant physiologists. IAA is a derivative of indole, containing a carboxymethyl substituent. It is a colorless solid that is soluble in polar organic solvents.

<span class="mw-page-title-main">Diquat</span> Chemical compound

Diquat is the ISO common name for an organic dication that, as a salt with counterions such as bromide or chloride is used as a contact herbicide that produces desiccation and defoliation. Diquat is no longer approved for use in the European Union, although its registration in many other countries including the USA is still valid.

<span class="mw-page-title-main">Hexazinone</span> Chemical compound

Hexazinone is an organic compound that is used as a broad spectrum herbicide. It is a colorless solid. It exhibits some solubility in water but is highly soluble in most organic solvents except alkanes. A member of the triazine class herbicides, it is manufactured by DuPont and sold under the trade name Velpar.

<span class="mw-page-title-main">Phenoxy herbicide</span> Class of herbicide

Phenoxy herbicides are two families of chemicals that have been developed as commercially important herbicides, widely used in agriculture. They share the part structure of phenoxyacetic acid.

<span class="mw-page-title-main">Fenoprop</span> Chemical compound

Fenoprop, also called 2,4,5-TP, is the organic compound 2-(2,4,5-trichlorophenoxy)propionic acid. It is a phenoxy herbicide and a plant growth regulator, an analog of 2,4,5-T in which the latter's acetic acid sidechain is replaced with a propionate group (with an extra CH3). The addition of this extra methyl group creates a chiral centre in the molecule and useful biological activity is found only in the (2R)-isomer. The compound's mechanism of action is to mimic the auxin growth hormone indoleacetic acid (IAA). When sprayed on plants it induces rapid, uncontrolled growth. As with 2,4,5-T, fenoprop is toxic to shrubs and trees.

<span class="mw-page-title-main">Dicamba</span> Chemical compound used as herbicide

Dicamba is a selective systemic herbicide first registered in 1967. Brand names for formulations of this herbicide include Dianat, Banvel, Diablo, Oracle and Vanquish. This chemical compound is a chlorinated derivative of o-anisic acid. It has been described as a "widely used, low-cost, environmentally friendly herbicide that does not persist in soils and shows little or no toxicity to wildlife and humans."

<span class="mw-page-title-main">Mecoprop</span> Chemical compound

Mecoprop 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. It is often used in combination with other chemically related herbicides such as 2,4-D, dicamba, and MCPA, which mimic the plant hormone IAA (auxin) and kill most broadleaf weeds by causing uncontrolled growth.

<span class="mw-page-title-main">Trifluralin</span> Chemical compound

Trifluralin is a commonly used pre-emergence herbicide. With about 14 million pounds (6,400 t) used in the United States in 2001, it is one of the most widely used herbicides. Trifluralin is generally applied to the soil to provide control of a variety of annual grass and broadleaf weed species. It inhibits root development by interrupting mitosis, and thus can control weeds as they germinate.

<span class="mw-page-title-main">Weed</span> Plant considered undesirable in a particular place or situation

A weed is a plant considered undesirable in a particular situation, growing where it conflicts with human preferences, needs, or goals. Plants with characteristics that make them hazardous, aesthetically unappealing, difficult to control in managed environments, or otherwise unwanted in farm land, orchards, gardens, lawns, parks, recreational spaces, residential and industrial areas, may all be considered weeds. The concept of weeds is particularly significant in agriculture, where the presence of weeds in fields used to grow crops may cause major losses in yields. Invasive species, plants introduced to an environment where their presence negatively impacts the overall functioning and biodiversity of the ecosystem, may also sometimes be considered weeds.

This is an index of articles relating to pesticides.

<span class="mw-page-title-main">Mesotrione</span> Chemical compound used as an herbicide

Mesotrione is a selective herbicide used mainly in maize crops. It is a synthetic compound inspired by the natural substance leptospermone found in the bottlebrush tree Callistemon citrinus. It inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD) and is sold under brand names including Callisto and Tenacity. It was first marketed by Syngenta in 2001.

<span class="mw-page-title-main">2,4-Dichlorophenoxyacetic acid</span> Herbicide

2,4-Dichlorophenoxyacetic acid is an organic compound with the chemical formula Cl2C6H3OCH2CO2H. It is usually referred to by its ISO common name 2,4-D. It is a systemic herbicide that kills most broadleaf weeds by causing uncontrolled growth, but most grasses such as cereals, lawn turf, and grassland are relatively unaffected.

4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors are a class of herbicides that prevent growth in plants by blocking 4-Hydroxyphenylpyruvate dioxygenase, an enzyme in plants that breaks down the amino acid tyrosine into molecules that are then used by plants to create other molecules that plants need. This process of breakdown, or catabolism, and making new molecules from the results, or biosynthesis, is something all living things do. HPPD inhibitors were first brought to market in 1980, although their mechanism of action was not understood until the late 1990s. They were originally used primarily in Japan in rice production, but since the late 1990s have been used in Europe and North America for corn, soybeans, and cereals, and since the 2000s have become more important as weeds have become resistant to glyphosate and other herbicides. Genetically modified crops are under development that include resistance to HPPD inhibitors. There is a pharmaceutical drug on the market, nitisinone, that was originally under development as an herbicide as a member of this class, and is used to treat an orphan disease, type I tyrosinemia.

<span class="mw-page-title-main">Quinclorac</span> Chemical compound

Quinclorac is an organic compound with the formula {C9NH4Cl2CO2H. A colorless solid, it is soluble in hydrocarbons and alcohols. The compound is the carboxylic acid of 3,7-dichloroquinoline.

<span class="mw-page-title-main">Imazaquin</span> Chemical compound

Imazaquin is an imidazolinone herbicide, so named because it contains an imidazolinone core. This organic compound is used to control a broad spectrum of weed species. It is a colorless or white solid, although commercial samples can appear brown or tan.

<span class="mw-page-title-main">Cyanazine</span> Chemical compound

Cyanazine is a herbicide that belongs to the group of triazines. Cyanazine inhibits photosynthesis and is therefore used as a herbicide.

<span class="mw-page-title-main">Fluazifop</span> ACCase herbicide, fop, anti-grass

Fluazifop is the common name used by the ISO for an organic compound that is used as a selective herbicide. The active ingredient is the 2R enantiomer at its chiral centre and this material is known as fluazifop-P when used in that form. More commonly, it is sold as its butyl ester, fluazifop-P butyl with the brand name Fusilade.

References

  1. ↑ Gimeno, Olga; Plucinski, Pawel; Kolaczkowski, Stan T.; Rivas, Francisco J.; Alvarez, Pedro M. (2003). "Removal of the Herbicide MCPA by Commercial Activated Carbons: Equilibrium, Kinetics, and Reversibility". Industrial & Engineering Chemistry Research. 42 (5): 1076–1086. doi:10.1021/ie020424x.
  2. ↑ Reade, J., Cobb, A. H. (2002). Herbicides: Modes of action and metabolism. Weed management handbook. pp. 157–158.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. 1 2 Grossmann, K. (2010). "Auxin herbicides: current status of mechanism and mode of action". Pest Management Science. 66 (2): 2033–2043. doi:10.1002/ps.1860. PMID   19823992.
  4. ↑ Templeman, W. G.; Marmoy, C. J. (November 1940). "The effect upon the growth of plants of watering with solutions of plant-growth substances and of seed dressings containing these materials". Annals of Applied Biology. 27 (4): 453–471. doi:10.1111/j.1744-7348.1940.tb07517.x.
  5. 1 2 3 Allen, H.P.; et al. (1978). "Chapter 5: Selective herbicides". In Peacock, F.C. (ed.). Jealott's Hill: Fifty years of Agricultural Research 1928-1978 . Imperial Chemical Industries Ltd. pp.  35–41. ISBN   0901747017.
  6. ↑ W.G. Templeman; W.A. Sexton (1946). "The Differential Effect of Synthetic Plant Growth Substances upon Plant Species. I. Seed Germination and Early Growth Responses to α-Naphthylacetic Acid and Compounds of General Formula arylOCHCOO". Proceedings of the Royal Society of London. 133 (872): 300–313. doi:10.1098/rspb.1946.0014. PMID   20994892.
  7. ↑ Troyer, James (2001). "In the beginning: the multiple discovery of the first hormone herbicides". Weed Science. 49 (2): 290–297. doi:10.1614/0043-1745(2001)049[0290:ITBTMD]2.0.CO;2.
  8. ↑ Slade, R.E.; Templeman, W.G.; Sexton, W.A. (1945). "Differential effects of plant growth substances on plant species". Nature. 155: 497–498. doi:10.1038/155497a0. S2CID   46259091.
  9. ↑ US Geological Survey (2020-06-18). "Estimated Agricultural Use for MCPA, 2017" . Retrieved 2020-09-01.
  10. ↑ "Chiptox". Nufarm. Retrieved 2020-09-01.
  11. 1 2 "MCPA". Extrnsion Toxicology Network. Cornell.
  12. ↑ J. Kobylecka; B. Ptaszynski; R. Rogaczewski; A. Turek (2003). "Phenoxyalkanoic acid complexes. Part I. Complexes of lead(II), cadmium(II) and copper(II) with 4-chloro-2-methylphenoxyacetic acid (MCPA)". Thermochimica Acta. 407 (1–2): 25–31. doi:10.1016/S0040-6031(03)00287-9.
  13. ↑ R. Kruszynski; T.J. Bartczak; B. Ptaszynski; A. Turek (2002). "A Novel Lead- bis (4-Chloro-2-Methylphenoxy)- Acetate Polymeric Complex". Journal of Coordination Chemistry. 55 (9): 1079–1089. doi:10.1080/0095897021000010035. S2CID   98738343.
  14. ↑ Prasad, Attaluri R.; Ramalingam, Thallapalli; Rao, Adari B.; Diwan, Prakash V.; Sattur, Pralhad B. (1989). "Synthesis and biological evaluation of 3-aryloxyalkyl-6-aryl-7H-s-triazolo[3,4-b][1,3,4]thiadiazines". European Journal of Medicinal Chemistry. 25 (2): 199–201. doi:10.1016/0223-5234(89)90116-5.
  15. 1 2 University of Hertfordshire (2016-10-17). "MCPA". sitem.herts.ac.uk. Retrieved 2016-11-21.
  16. 1 2 Helweg, A. (1987). "Degradation and adsorption of 14C-MCPA in soil—influence of concentration, temperature and moisture content on degradation". Weed Research. 27 (4): 287–296. doi:10.1111/j.1365-3180.1987.tb00765.x.
  17. ↑ "Ecotoxicology of MCPA". Pesticide Properties DataBase. University of Hertfordshire. Retrieved 2016-10-17.
  18. 1 2 UNEP publications, OECD SIDS, 4-chloro-2-methylphenol. http://www.inchem.org/documents/sids/sids/1570645.pdf
  19. ↑ Guidelines for drinking-water quality, 2nd ed. Vol.2. Health criteria and other supporting information. World Health Organization, Geneva, 1996.
  20. ↑ Bala, Tanushree; Prasad, B. L. V.; Sastry, Murali; Kahaly, Mousumi Upadhyay; Waghmare, Umesh V. (2007-07-19). "Interaction of different metal ions with carboxylic acid group: a quantitative study". The Journal of Physical Chemistry A. 111 (28): 6183–6190. Bibcode:2007JPCA..111.6183B. doi:10.1021/jp067906x. ISSN   1089-5639. PMID   17585841.
  21. 1 2 Zertal, A.; Molnár-Gábor, D.; Malouki, M.A.; Sehili, T.; Boule, P. (2004-05-01). "Photocatalytic transformation of 4-chloro-2-methylphenoxyacetic acid (MCPA) on several kinds of TiO2". Applied Catalysis B: Environmental. 49 (2): 83–89. doi:10.1016/j.apcatb.2003.11.015. ISSN   0926-3373.
  22. ↑ Bælum, Jacob; Nicolaisen, Mette H.; Holben, William E.; Strobel, Bjarne W.; Sørensen, Jan; Jacobsen, Carsten S. (2008-03-20). "Direct analysis of tfdA gene expression by indigenous bacteria in phenoxy acid amended agricultural soil". The ISME Journal. 2 (6): 677–687. doi: 10.1038/ismej.2008.21 . ISSN   1751-7362. PMID   18356824.
  23. ↑ Nielsen, Morten S.; Bælum, Jacob; Jensen, Malene B.; Jacobsen, Carsten S. (2011-05-01). "Mineralization of the herbicide MCPA in urban soils is linked to presence and growth of class III tfdA genes". Soil Biology and Biochemistry. 43 (5): 984–990. doi:10.1016/j.soilbio.2011.01.014.
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