Carbamate Insecticides
Carbamates are considered slightly less toxic than organophosphates since they are rapidly metabolized by the body and excreted. Carbamates are compounds based upon carbamic acid with various hydrocarbon groups substituted for H on the molecule. They also do not persist in the environment for as long as organophosphates, usually breaking down in a couple of days to a week. Carbamates generally have a low vapor pressure and low water solubility, meaning they are slow to evaporate and will not dissolve readily in water.
Carbamates act mainly as contact and oral poisons as they are absorbed readily through the skin, stomach lining or respiratory tract. They have a similar mode of action as organophosphates which are to inhibit the functioning of acetylcholinesterase, resulting in nervous system failure. Carbamates are considered to be reversible inhibitors, which mean recovery from over exposure is typically faster than with organophosphates. They do not remain in the body like other pesticides can, minimizing the poisoning risk.
Carbamates are considered moderately toxic. Acute poisoning happens within minutes of exposure and will last only a few hours as the body metabolizes the chemical. Symptoms include stomach cramps and sweating; if exposure continues, symptoms will mimic organophosphate poisoning with slurring of speech, twitching and jerky movements, and possibly dizziness and blurred vision. Chronic poisoning is not common since the body is able to metabolize and excrete carbamates.
The most widely used carbamate insecticide is Sevin (Carbaryl) which is used to kill insects on lawns or gardens and, because of its low toxicity to mammals, can even be sprinkled on pets! There are concerns through about its broad spectrum application for chewing insects. It also kills beneficial insects such as predator insects and the pollinators.
Green chemistry has advanced this class of chemicals with a new synthesis of carbamates. Carbamates are used for herbicides, pesticides, and fungicides, as well as pharmaceuticals and the manufacturing of polymers, especially polyurethanes. The existing technology to produce them was based upon the use of very toxic reagents and intermediates. Many of these products used phosgene as the starting raw material and now through green chemistry alternative ways of producing these chemicals have been discovered. Recently an alternative approach was proposed for synthesis of carbamates that is based on the use of substituted carbonates or ureas as the reagents in the presence of solid basic catalysts (silica gel, the sodium form of zeolite ZSM-5, Li-MgO, Mg-Al hydrocalcite). There has also been an electrochemical method of synthesis of carbamates from CO 2, oxygen, and amines.
Commonly used carbamate insecticides to use for review of the labels and MSDS sheets, to fill in the product profile worksheet at the end of the unit and to discuss with the students include Carzol (Formetanate hydrochloride), Furadan (Carbofuran), Sevin (Carbaryl), and Temik (Aldicarb). Students should contrast and compare the different insecticides and determine the use limitations of each of the chemicals. Their Excel sheet should be filled out for each of these chemicals with their half-life in the environment, water solubility, partition coefficient (K O C), and calculated GUS.
Comments: