Metaldehyde is obtained in moderate yields by treatment of acetaldehyde with various acid catalysts, such as hydrogen bromide. The reaction is reversible; upon heating to about 80 °C, metaldehyde reverts to acetaldehyde. Metaldehyde exists as a mixture of stereoisomers, molecules that differ with respect to the relative orientation of the methyl groups on the 8-membered ring.
Uses
As a pesticide
It is sold under various trade names as a molluscicide, including Antimilice, Ariotox, Blitzem, Cekumeta, Deadline, Defender, Halizan, Limatox, Limeol, Meta, Metason, Mifaslug, Namekil, Slug Fest Colloidel 25, and Slugit. Typically it is applied in the form of slug pellets, which normally include a wheat bait. Metaldehyde acts on the pest by contact or ingestion, and the aqueous environment inside the pest's cells readily hydrolyzes metaldehyde into acetaldehyde, the molecule associated with an alcohol hangover. 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 ". On 19 December 2018, the British government banned the use of metaldehyde slug pellets outdoors from spring 2020; after this date it would only be legal to use it in permanent greenhouses. In July 2019, the ban was overturned after the High Court in London agreed with a challenge to its legality. Metaldehyde pellets returned to the UK market for the foreseeable future.
Other uses
Metaldehyde is also used as a camping fuel, also for military purposes, or solid fuel in lamps. It may be purchased in a tablet form to be used in small stoves, and for preheating of Primus type stoves. It is sold under the trade name of "META" by Lonza Group of Switzerland; it can be included in the field ration of some nations.
Safety and toxicity to pets
Metaldehyde has a toxicity profile identical to that for acetaldehyde, being mildly toxic and a respiratory irritant at the 50 ppm level. In terms of water safety, during periods of rainfall metaldehyde pellets become agitated and can seep into natural water courses. The European Commission restricts metaldehyde levels to 0.1 µg/L in drinking water. Metaldehyde-containing slug baits should be used with caution because they are toxic to dogs and cats. If ingested by dogs or cats, tremors, drooling, and restlessness will proceed to seizures and death within hours to days if treatment is not started quickly. Due to this toxicity, pet owners may want to investigate alternatives which are not as toxic to pets, such as Ferric Sodium EDTA or Iron phosphate. The metaldehyde tablets resemble sweetmeats and do not taste bad, making accidental ingestion possible by children or even by adults unaware of their true nature. Their use was popular during the inter-war period and several cases of poisoning resulted. Baits may contain a bittering agent to prevent accidental consumption by pets or children.
