MCPA

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.

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.

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:

: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.

MCPA was first reported in the open scientific literature by Slade, Templeman and Sexton in 1945. 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.

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". Though not extremely toxic, it has been determined that MCPA can form complexes with metal ions and thereby increase their bioavailability, and there is also work being done to utilize this ability.

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.

Brand names

The following commercial products contain MCPA: 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. 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. 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.

The carboxyl group of MCPA can form conjugated complex with metals as a ligand. 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

-COOH + M⁺ → -COOM + H⁺

Bio-degradation

thumb|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.

Photo-degradation

thumb|Oxidation of MCPA by hydroxyl radicals

thumb|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, CH<small>2</small>-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.