Carbaryl

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Carbaryl

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UN number	2757
Template:Longitem	Template:Chembox Elements/molecular formula
Molar mass	Template:Chem molar mass
Appearance	Colorless crystalline solid
Density	1.2 g/cm3
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Template:Longitem	QP53AE01 (WHO)
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Spraying carbaryl on pine trees

Carbaryl (1-naphthyl methylcarbamate) is an organic compound with the formula Template:Chem2. Classified as a carbamate, it is used chiefly as an insecticide. It is a white solid was under the brand name Sevin, which was a trademark of the Bayer Company.

Production

Carbaryl is often inexpensively produced by direct reaction of methyl isocyanate with 1-naphthol.

C₁₀H₇OH + CH₃NCO → C₁₀H₇OC(O)NHCH₃

Alternatively, 1-naphthol can be treated with excess phosgene to produce 1-naphthylchloroformate, which is then converted to carbaryl by reaction with methylamine. The former process was used in the chemical plant that caused the Bhopal disaster. In comparison, the latter synthesis uses exactly the same reagents required for the synthesis of methyl isocyanate. This route avoids the potential hazards of methyl isocyanate, allbeit at a higher cost.

Biochemistry

Carbamate insecticides are reversible inhibitors of the enzyme acetylcholinesterase and are considered less toxic than organophosphate insecticides, which inhibit the enzyme irreversibly. They resemble acetylcholine, but the carbamoylated enzyme undergoes the final hydrolysis step very slowly (minutes) compared with the acetylated enzyme generated by acetylcholine (microseconds). They interfere with the cholinergic nervous system and cause death because the effects of the neurotransmitter acetylcholine cannot be terminated by carbamoylated acetylcholinesterase.

Applications

Union Carbide discovered carbaryl and commercialized it in 1958. Bayer purchased Aventis CropScience in 2002, a company that included Union Carbide pesticide operations. Carbaryl was once the third-most-used insecticide in the United States for home gardens, commercial agriculture, and forestry and rangeland protection. As a veterinary drug, it is known as carbaril (INN).

The commercial introduction of the carbamate insecticides in 1959 was hailed as a major breakthrough in pesticides. The carbamates do not have the persistence of chlorinated pesticides. Although toxic to insects, carbaryl is detoxified and eliminated rapidly in vertebrates. It is neither concentrated in fat nor secreted in milk, so is favored for food crops, at least in the US. During the period 1992-2001, when it was in peak use, 3.9M pounds were sold. Since 2007, its use was cancelled in UK and Europe, and it remain permitted in US, Canada, and Australia. Even where it is allowed, it use has been curtailed.

Ecology

Carbaryl kills both targeted (e.g., malaria-carrying mosquitos) and beneficial insects (e.g., honeybees), as well as crustaceans. Because it is highly toxic to zooplankton, the algae they feed on experience blooms. Boone & Bridges 2003 find that larger algae eaters such as Bufo woodhousii benefit from this effect.

Although it can increase survival rates for organisms that benefit from consequential algae blooms, the long term effects of these changes in competition levels and predation can be detrimental to many aquatic ecosystems, particularly those dominated by Anurans.

It is approved for more than 100 crops in the US. Carbaryl has been illegal in the EU since 21 November 2007 and Angola.

Safety

Carbaryl is a cholinesterase inhibitor and is toxic to humans. It is classified as a likely human carcinogen by the United States Environmental Protection Agency (EPA.) The oral LD₅₀ is 250 to 850 mg/kg for rats and 100 to 650 mg/kg for mice.

Carbaryl can be produced using methyl isocyanate (MIC) as an intermediary. A leak of MIC used in the production of carbaryl caused the Bhopal disaster, the most lethal industrial accident in history.

External links

• Carbaryl Technical Fact Sheet - National Pesticide Information Center
• Carbaryl Pesticide Information Profile - Extension Toxicology Network
• Cholinesterase Inhibition - Extension Toxicology Network
• EPA info
• EPA factsheet
• IPCS (WHO) Health and Safety Guide
• Environmental Health Criteria - WHO
• Exclusive Chemistry Ltd - routes of Sevin synthesis
• CDC - NIOSH Pocket Guide to Chemical Hazards
• Template:PPDB