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
enzyme inhibitor of
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.[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] 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 methoxy
acetone. The resulting
imine is
hydrogenation to give primarily the
S-stereoisomeric amine. This
secondary amine is acetylated with chloroacetylchloride. Because of the
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
. Both atropisomers of (
S)-metolachlor exhibit the same biological activity.
Safety and ecological effects
The European Chemicals Agency classified metolachlor as a suspected human carcinogen (Carcinogen category 2) in 2022.
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).
[USEPA,1987. Metolachlor Pesticide Registration Standard. Springfield, IL: Natl. Tech. Info. Serv.] 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.
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.
Metolachlor induces cytotoxic and genotoxic effects in human lymphocytes.[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.] and chicken embryos exposed to metolchlor showed a significant decrease in the average body mass compared to the control.[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.]
See also
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