Metolachlor

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. 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.

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). 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. Genotoxic effects have also been observed in tadpoles exposed to metolachlor. Evidence also reveals that metolachlor affects cell growth. Cell division in yeast was reduced, and chicken embryos exposed to metolchlor showed a significant decrease in the average body mass compared to the control. need a secondary citation: === Fate in the environment === Metabolites of metolachlor have been found in varying levels treated plants. Plants however, retain their metolachlor metabolites although animals that consume such plants are able to break down and eliminate the chemical rapidly. Some parts of plants, such as the leaves of cotton can retain much higher levels of metolachlor residues compared to other parts of the plants such as the seeds that can contain little.

The breakdown of metolachlor in the soil is affected by a number of factors such as moisture, temperature, microbial activity, soil type , concentration of oxygen and nitrification and is mobile and easily leached through soil. The effect of changes in moisture content and temperature related to microbial activity also affects metolachlor breakdown.--

References

References

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