Carbaryl is a chemical in the carbamate family used chiefly as an insecticide. It is a white
crystalline solid commonly sold under the brand name Sevin, a trademark of the Bayer
Company. Union Carbide discovered carbaryl and introduced it commercially in 1958. Bayer
purchased Aventis CropScience in 2002, a company that included Union Carbide pesticide operations.
It remains the third-most-used insecticide in the United States for home gardens, commercial
agriculture, and forestry and rangeland protection. About 11 million kilograms were applied to
U.S. farm crops in 1976. As a veterinary drug, it is known as carbaril.
Production Carbaryl is produced by treating methyl isocyanate
with 1-naphthol.
Alternatively, 1-naphthol is first converted to its chloroformate, which is then treated
with methylamine to give the desired product:
The former process was carried out in Bhopal. In comparison, the latter synthesis uses exactly
the same reagents, but in a different sequence. This synthesis avoids the preparation of methyl
isocyanate, and the comparison is often used as a case study in green chemistry. That said,
the second synthesis still uses phosgene, which is toxic and reacts with water, and
methylamine, which is a toxic and flammable gas.
Biochemistry Carbamate insecticides are slowly reversible
inhibitors of the enzyme acetylcholinesterase. They resemble acetylcholine, but the carbamoylated
enzyme undergoes the final hydrolysis step very slowly compared with the acetylated enzyme
generated by acetylcholine. They interfere with the cholinergic nervous system and cause
death because the effects of the neurotransmitter acetylcholine cannot be terminated by carbamoylated
acetylcholinesterase. Applications
The development of the carbamate insecticides has been called 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. It is
the active ingredient in Carylderm shampoo used to combat head lice until infestation
is eliminated. Risk
Carbaryl kills both targeted and beneficial insects, as well as crustaceans.
Although approved for more than 100 crops in the US, carbaryl is illegal in several
countries, including the United Kingdom, Austria, Denmark, Sweden, Iran, Germany, and Angola.
Carbaryl is often produced using methyl isocyanate as an intermediary. A leak of MIC used in
the production of carbaryl caused the Bhopal disaster, the largest industrial accident
in history. This accident caused around 11,000 deaths and over 500,000 injuries.
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 The oral LD50 is 250 to 850 mg/kg for rats and 100 to 650 mg/kg
for mice. References
^ a b Robert L. Metcalf “Insect Control” in Ullmann’s Encyclopedia of Industrial
Chemistry” Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a14_263 ^ a b Thomas A. Unger. Pesticide synthesis
handbook. William Andrew. pp. 67–68. ISBN 0-8155-1401-8. ^ Carbaryl Insecticide Hazard Data
^ Interim Reregistration Eligibility Decision for Carbaryl, U.S. EPA, June 2003.
External links Carbaryl Technical Fact Sheet - National Pesticide
Information Center Carbaryl General Fact Sheet - National Pesticide
Information Center Carbaryl Pesticide Information Profile - Extension
Toxicology Network Cholinesterase Inhibition - Extension Toxicology
Network EPA info
EPA factsheet IPCS Health and Safety Guide
Environmental Health Criteria - WHO Exclusive Chemistry Ltd - routes of Sevin
synthesis CDC - NIOSH Pocket Guide to Chemical Hazards
Pesticide Properties Database record for Carbaryl