Wednesday, July 25, 2007

Organophosphate_Types of Pesticides

One of the products of the reaction of Fc2P2S4 with dimethyl cyanamide

An organophosphate (sometimes abbreviated OP) is the general name for esters of phosphoric acid and is one of the organophosphorus compounds. They can be found as part of insecticides, herbicides, and nerve gases, amongst others. Some less-toxic organophosphates can be used as solvents, plasticizers, and EP additives.
Early pioneers in the field include Jean Louis Lassaigne (early 1800s) and Philip de Clermount (1854). In 1932, German chemist Willy Lange and his graduate student, Gerde von Krueger, first described the cholinergic nervous system effects of organophosphates, noting a choking sensation and a dimming of vision after exposure. This discovery later inspired German chemist Gerhard Schrader at company I.G. Farben in the 1930s to experiment with these compounds as insecticides. Their potential use as chemical warfare agents soon became apparent, and the Nazi government put Schrader in charge of developing organophosphate (in the broader sense of the word) nerve gases. Schrader's laboratory discovered the G series of weapons, which included Sarin, Tabun, and Soman. The Nazis produced large quantities of these compounds, though did not use them during World War II (likely because they feared the Allies possessed similar weapons). British scientists experimented with an anticholinergic organophosphate of their own, called diisopropylfluorophosphate (DFP), during the war. The British later produced VX nerve gas, which was many times more potent than the G series, in the early 1950s.
After World War II, American companies gained access to some information from Schrader's laboratory, and began synthesizing organophosphate pesticides in large quantities. Parathion was among the first marketed, followed later by malathion and azinphosmethyl. The popularity of these insecticides increased after many of the organochlorine insecticides like DDT, dieldrin, and heptachlor were banned in the 1970s.

Organophosphate pesticides
In health, agriculture, and government, the word "organophosphates" refers to a group of insecticides or nerve agents acting on the enzyme acetylcholinesterase (the pesticide group Carbamates also act on this enzyme, but through a different mechanism). Organophosphate pesticides (as well as Sarin and VX nerve gas) irreversibly inactivate acetylcholinesterase, which is essential to nerve function in insects, humans, and many other animals. Organophosphate pesticides have tremendous variation in their ability to affect this enzyme, and thus in their potential for poisoning. For instance, parathion, one of the first OPs discovered, is many times more potent than malathion, an insecticide used in combatting the Mediterranean fruit fly (Med-fly) and West Nile Virus-transmitting mosquitoes.
Organophosphate pesticides tend to degrade rapidly on exposure to sunlight, air, and soil, though small amounts can persist and end up in food and drinking water. Their ability to degrade made them an attractive alternative to the persistent organochlorine pesticides, such as DDT, aldrin, and dieldrin, which were widely publicized in Rachel Carson's Silent Spring. While organophosphates degrade faster than the organochlorines, they have much greater acute toxicity, posing risks to farmworkers, pesticide applicators, and anyone else who may be exposed to large amounts. OP poisoning can be very serious and even cause death. See the Toxicity section below for the effects. Their toxicity is not limited to the acute phase, however, and chronic effects have long been noted. Neurotransmitters such as acetylcholine (which is affected by organophosphate pesticides) are profoundly important in the brain's development, and many OPs have neurotoxic effects on developing organisms even from low levels of exposure.
Commonly used organophosphates have included Parathion, Malathion, Methyl parathion, Chlorpyrifos, Diazinon, Dichlorvos, Phosmet, Azinphos methyl.

A good example of this chemistry are the P-thiocyanate compounds which use an aryl (or alkyl) group and an alkylamino group as the lipophilic groups. The thiocyanate is the leaving group.

It was claimed in a German patent that the reaction of 1,3,2,4-dithiadiphosphetane 2,4-disulfides with dialkyl cyanamides formed plant protection agents which contained six membered (P-N=C-N=C-S-) rings. It has been proven in recent times by the reaction of diferrocenyl 1,3,2,4-dithiadiphosphetane 2,4-disulfide (and Lawesson's reagent) with dimethyl cyanamide that in fact a mixture of several different phosphorus-containing compounds is formed. Depending on the concentration of the dimethyl cyanamide in the reaction mixture either a different six membered ring compound (P-N=C-S-C=N-) or a nonheterocylic compound (FcP(S)(NR2)(NCS)) is formed as the major product, the other compound is formed as a minor product.
In addition small traces of other compounds are also formed in the reaction. It is unlikely that the ring compound (P-N=C-S-C=N-) {or its isomer} would act as a plant protection agent, but (FcP(S)(NR2)(NCS)) compounds can act as nerve poisons in insects.

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