Metaldehyde

Metaldehyde is an organic compound with the formula (C₈H₁₆O₄). It is used as a pesticide against slugs and snails. It is the cyclic tetramer of acetaldehyde.

Metaldehyde is flammable, toxic if ingested in large quantities, and irritating to the skin and eyes. It has a white crystalline appearance with a menthol odor.

Metaldehyde is obtained in moderate yields by treatment of acetaldehyde with chilled mineral acids. The liquid trimer, paraldehyde is also obtained. The reaction is reversible; upon heating to about 80 °C, metaldehyde and paraldehyde revert to acetaldehyde.

Metaldehyde exists as a mixture of four stereoisomers, molecules that differ with respect to the relative orientation of the methyl groups on the 8-membered ring. The stereoisomers have respectively the molecular symmetries C_s (with symmetry of order 2), C_2v (order 4), D_2d (order 8), and C_4v (order 8). All have at least one plane of reflexion, so none of them is chiral.

It is sold under various trade names as a molluscicide, including Antimilice, Ariotox, Blitzem (in Australia), Cekumeta, Deadline, Defender (in Australia), Halizan, Limacide, Limatox, Limeol, Meta, Metason, Mifaslug, Namekil, Slug Fest, and Slugit. Typically it is applied in pellet form, but it is also found as a liquid spray, granules, paste, or dust. Often the pesticide includes bran or molasses to attract pests, making it attractive to household pets as well.

Metaldehyde is effective on pests by contact or ingestion and works by limiting the production of mucus in mollusks making them susceptible to dehydration.

Metaldehyde products were used to control the invasive African land snail population in Miami-Dade County in Florida. Experimental use permits from the U.S. Environmental Protect Agency authorized the application amount and usage in residential areas.

Due to the contamination of drinking water by metaldehyde's use in agriculture, a specialist organisation was established in 2008 called "The Metaldehyde Stewardship Group (MSG)".