Organophosphate



An organophosphate (sometimes abbreviated OP) is the general name for plasticizers, and EP additives.

Organophosphates are widely employed both in natural and synthetic applications because of the ease with which organic groups can be linked together. Being a triprotic acid, phosphoric acid can form triesters whereas carboxylic acids only form monoesters. Esterification entails the attachment of organic groups to phosphorus through oxygen linkers. The precursors to such esters are alcohols. Encompassing many thousands of natural and synthetic compounds, alcohols are diverse and widespread.

OP(OH)3 + ROH → OP(OH)2(OR) + H2O
OP(OH)2(OR) + R'OH → OP(OH)(OR)(OR') + H2O
OP(OH)(OR)(OR') + R"OH → OP(OR)(OR')(OR") + H2O

The phosphate esters bearing OH groups are ATP, which is the monoester of triphosphoric acid (H5P3O10).

Alcohols can be detached from phosphate esters by hydrolysis, which is the reverse of the above reactions. For this reason, phosphate esters are common carriers of organic groups in biosynthesis.

Organophosphate pesticides

In health, agriculture, and government, the word "organophosphates" refers to a group of insecticides or nerve agents acting on the enzyme phosphinic acid are used as organic phosphorus containing neurotoxin.

Organophosphate pesticides (as well as parathion, one of the first OPs commercialized, 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 degrade rapidly by hydrolysis on exposure to sunlight, air, and soil, although small amounts can be detected in food and drinking water. Their ability to degrade made them an attractive alternative to the persistent organochlorine pesticides, such as dieldrin. Although organophosphates degrade faster than the organochlorines, they have greater acute toxicity, posing risks to people who may be exposed to large amounts (see the Toxicity section below).

Commonly used organophosphates have included Azinphos methyl.

Organophosphate neurotoxins

History of nerve gases

Early pioneers in the field include 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. dieldrin, and heptachlor were banned in the 1970s.

Structural features of organophosphate neurotoxins

Effective organophosphate neurotoxins have the following structural features:

  • A terminal oxygen connected to phosphorus by a double bond, i.e. a phosphoryl group
  • Two lipophilic groups bonded to the phosphorus
  • A halide

Terminal oxygen vs. terminal sulfur

Thiophosphoryl compounds, those bearing the P=S functionality, are much less toxic than related phosphoryl derivatives, which includes as lipophilic side groups from the phosphorus atom while an insect tends to oxidise the compound so removing the terminal sulfur and replacing it with a terminal oxygen which causes the compound to be more able to act as an acetylcholinesterase inhibitor.

Fine tuning

Within these requirements, a large number of different lipophilic and leaving groups have been used. The variation of these groups is one means of fine tuning the toxicity of the compound. A good example of this chemistry are the P-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.

Organophosphate poisoning

Main article: Organophosphate poisoning

Many organophosphates are potent 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.

See also

References

  • Costa LG. Current issues in organophosphate toxicology. Clin Chim Acta. 2006 Apr;366(1-2):1-13. Epub 2005 December 6. Review. PMID: 16337171.
 
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