Metolachlor Explained

Metolachlor is an organic compound that is widely used as an herbicide. It is a derivative of aniline and is a member of the chloroacetanilide family of herbicides. It is highly effective toward grasses.

Agricultural use

Metolachlor was developed by Ciba-Geigy. Its acts by inhibition of elongases and of the geranylgeranyl pyrophosphate (GGPP) cyclases, which are part of the gibberellin pathway. It is used for grass and broadleaf weed control in corn, soybean, peanuts, sorghum, and cotton. It is also used in combination with other herbicides.

Metolachlor is a popular herbicide in the United States.[1] As originally formulated metolachlor was applied as a racemate, a 1:1 mixture of the (S)- and (R)-stereoisomers. The (R)-enantiomer is less active, and modern production methods afford a higher concentration of S-metolachlor, thus current application rates are far lower than original formulations.

Production and basic structure

Metolachlor is produced from 2-ethyl-6-methylaniline (MEA) via condensation with methoxyacetone. The resulting imine is hydrogenated to give primarily the S-stereoisomeric amine. This secondary amine is acetylated with chloroacetylchloride. Because of the steric effects of the 2,6-disubstituted aniline, rotation about the aryl-C to N bond is restricted. Thus, both the (R)- and the (S)-enantiomers exist as atropisomers. Both atropisomers of (S)-metolachlor exhibit the same biological activity.[2]

Safety and ecological effects

The European Chemicals Agency classified metolachlor as a suspected human carcinogen (Carcinogen category 2) in 2022.[3] The United States Environmental Protection Agency (US EPA) has classified Metolachlor as a Group C, possible human carcinogen, based on liver tumors in rats at the highest dose tested (HDT).[4] [5] Evidence of the bioaccumulation of metolachlor in edible species of fish as well as its adverse effect on the growth and development has raised concerns on its effects on human and environmental health. For example, products with this active ingredient are restricted to professional licensed applicators in the U.S. state of Massachusetts.[6] Though there is no set maximum concentration (maximum contaminant level, MCL) for metolachlor that is allowed in drinking water, the US EPA does have a health advisory level (HAL) of 0.525 mg/L. Metolachlor has been detected in ground and surface waters in concentrations ranging from 0.08 to 4.5 parts per billion (ppb) throughout the U.S.[7]

Metolachlor induces cytotoxic and genotoxic effects in human lymphocytes.[8] Genotoxic effects have also been observed in tadpoles exposed to metolachlor.[9] Evidence also reveals that metolachlor affects cell growth. Cell division in yeast was reduced,[10] and chicken embryos exposed to metolchlor showed a significant decrease in the average body mass compared to the control.[11]

See also

Notes and References

  1. Kiely, T., D. Donaldson, and A. Grube. 2004. Pesticide industry sales and usage: 2000 and 2001 market estimates. US Environmental Protection Agency, Office of Pesticides Programs, Washington, DC
  2. H.U.-Blaser . The Chiral Switch of (S)-Metolachlor: A Personal Account of an Industrial Odyssey in Asymmetric Catalysis . Advanced Synthesis and Catalysis . 2002 . 344 . 17–31 . 10.1002/1615-4169(200201)344:1<17::AID-ADSC17>3.0.CO;2-8.
  3. Alvarez . Fernando . Arena . Maria . Auteri . Domenica . Binaglia . Marco . Castoldi . Anna Federica . Chiusolo . Arianna . Colagiorgi . Angelo . Colas . Mathilde . Crivellente . Federica . De Lentdecker . Chloe . De Magistris . Isabella . Egsmose . Mark . Fait . Gabriella . Ferilli . Franco . Gouliarmou . Varvara . Nogareda . Laia Herrero . Ippolito . Alessio . Istace . Frederique . Jarrah . Samira . Kardassi . Dimitra . Kienzler . Aude . Lanzoni . Anna . Lava . Roberto . Leuschner . Renata . Linguadoca . Alberto . Lythgo . Christopher . Magrans . Oriol . Mangas . Iris . Miron . Ileana . Molnar . Tunde . Padovani . Laura . Panzarea . Martina . Parra Morte . Juan Manuel . Rizzuto . Simone . Serafimova . Rositsa . Sharp . Rachel . Szentes . Csaba . Szoradi . Andras . Terron . Andrea . Theobald . Anne . Tiramani . Manuela . Vianello . Giorgia . Villamar‐Bouza . Laura . Peer review of the pesticide risk assessment of the active substance S‐metolachlor excluding the assessment of the endocrine disrupting properties . EFSA Journal . February 2023 . 21 . 2 . 10.2903/j.efsa.2023.7852. 9972551 .
  4. Web site: S-metolachlor; Pesticide Tolerance . Federal Register . US EPA . 15 October 2024 . August 30, 2006.
  5. USEPA,1987. Metolachlor Pesticide Registration Standard. Springfield, IL: Natl. Tech. Info. Serv.
  6. Web site: Massachusetts Department of Agricultural Resources . Groundwater Protection List . Commonwealth of Massachusetts . 15 October 2024.
  7. Pothuluri, J.V., Evans, F.E., Doerge, D.R., Churchwell, M.I.,Cerniglia, C.E. . 1997 . Metabolism of metolachlor by the fungus Cunninghamella elegans . Arch. Environ. Contam. Toxicol. . 32 . 117–125 . 10.1007/s002449900163 . 9069185 . 2. 20614148 .
  8. Rollof, B., Belluck, D., Meiser, L. . 1992 . Cytogenic effects of cyanazine and metolachlor on human lymphocytes exposed in vitro . Mutat. Res. Lett. . 281 . 295–298 . 10.1016/0165-7992(92)90024-C . 1373225 . 4. free .
  9. Clements, C., Ralph, S.,Petras, M. . 1997 . Genotoxicity of select herbicides on Rana catesbeiana tadpoles using alkaline single-cell gel DNA electrophoresis (Comet) assay . Env. Mol. Mut. . 29 . 277–288 . 10.1002/(SICI)1098-2280(1997)29:3<277::AID-EM8>3.0.CO;2-9 . 3. 9142171 . 27619855 .
  10. Echeverrigaray,S., Gomes,L.H., Taveres, F.C.A.(1999). Isolation and characterization of metolachlor resistant mutants of Saccharomyces cerevisiae. World Journal of Micro and Biotech. 15: 679–681.
  11. Varnargy,L., Budai, P., Fejes, S., Susan, M., Francsi, T., Keseru, M., Szabo, R.(2003). Toxicity and degradation of metolachlor (Dual 960EC) in chicken embryos. Commun. Agric. Appl. Biol. Sci.68:807–11.

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