Parathion

Handling properties

It is commonly distributed as a brown liquid that smells of rotting eggs or garlic. The insecticide is somewhat stable, although it darkens when exposed to sunlight.

Industrial synthesis

Parathion is synthesized from diethyl dithiophosphoric acid (C₂H₅O)₂PS₂H by chlorination to generate diethylthiophosphoryl chloride ((C₂H₅O)₂P(S)Cl), and then the chloride is treated with sodium 4-nitrophenolate (the sodium salt of 4-nitrophenol).

2 (C₂H₅O)₂P(S)SH + 3 Cl₂ → 2 (C₂H₅O)₂P(S)Cl + S₂Cl₂ + 2 HCl

(C₂H₅O)₂P(S)Cl + NaOC₆H₄NO₂ → (C₂H₅O)₂P(S)OC₆H₄NO₂ + NaCl

Applications

As a pesticide, parathion is generally applied by spraying. It is often applied to cotton, rice and fruit trees. The usual concentrations of ready-to-use solutions are 0.05 to 0.1%. The chemical is banned for use on many food crops.

Insecticidal activity

Parathion acts on the enzyme acetylcholinesterase indirectly. After an insect (or a human) ingests parathion, an oxidase replaces the double bonded sulfur with oxygen to give paraoxon.

(C₂H₅O)₂P(S)OC₆H₄NO₂ +⁠1/2⁠O₂ → (C₂H₅O)₂P(O)OC₆H₄NO₂ + S

The phosphate ester is more reactive in organisms than the phosphorothiolate ester, as the phosphorus atoms become much more electropositive.

Parathion resistance is a special case of acetylcholinesterase inhibitor resistance.

Parathion is toxic to most forms of wildlife in addition to insects if not applied at correct concentrations.

Degradation

Degradation of parathion leads to more water-soluble products. Hydrolysis, which deactivates the molecule, occurs at the aryl ester bond resulting in diethyl thiophosphate and 4-nitrophenol:

(C₂H₅O)₂P(S)OC₆H₄NO₂ + H₂O → HOC₆H₄NO₂ + (C₂H₅O)₂P(O)SH

Degradation proceeds differently under anaerobic conditions: the nitro group on parathion is reduced to the amine:

(C₂H₅O)₂P(S)OC₆H₄NO₂ + 6 H → (C₂H₅O)₂P(S)OC₆H₄NH₂ + 2 H₂O

Illicit uses

As a chemical weapon

Parathion was used as a chemical warfare agent, most notably by an element of the British South Africa Police attached to the Selous Scouts during the Rhodesian Bush War. They used it to poison clothing that was then supplied to anti-government guerrillas. When the enemy soldiers put on the clothes, they were poisoned by absorption through the skin.

Use in suicides

Parathion was commonly used for suicides in the 1950s and 1960s.

Safety

Parathion disrupts the nervous system by inhibiting acetylcholinesterase. It is absorbed via skin, mucous membranes, and orally. Absorbed parathion is rapidly metabolized to paraoxon. Exposure can result in headaches, convulsions, poor vision, vomiting, abdominal pain, severe diarrhea, unconsciousness, tremor, dyspnea, and finally pulmonary edema as well as respiratory arrest. Symptoms of poisoning are known to last for extended periods, sometimes months. The most common and very specific antidote is atropine, in doses of up to 100 mg daily. Because atropine may also be toxic, it is recommended that small frequently repeated doses be used in treatment. If human poisoning is detected early and the treatment is prompt (atropine and artificial respiration), fatalities are infrequent. Insufficient oxygen will lead to cerebral hypoxia and permanent brain damage. Peripheral neuropathy including paralysis is noticed as late sequelae after recovery from acute intoxication.

Based on animal studies, parathion is considered by the U.S. Environmental Protection Agency to be a possible human carcinogen. Studies show that parathion is toxic to fetuses, but does not cause birth defects.

It is classified by the United Nations Environment Programme as a persistent organic pollutant and by the World Health Organization as Toxicity Class Ia (extremely hazardous).