Parathion

Parathion, also called parathion-ethyl or diethyl parathion, is an organophosphate compound. It is a potent insecticide and acaricide. It was originally developed by IG Farben in the 1940s. It is highly toxic to non-target organisms, including humans. Its use is banned or restricted in many countries, and there are proposals to ban it from all use. Closely related is "methyl parathion" (see below).

Parathion-methyl
"Parathion-methyl (CAS#298-00-0) was also developed and is marketed for similar uses. It is a distinct compound with diminished toxicity. Other names for parathion-methyl include methyl parathion and dimethyl parathion; some trade names include Bladan M, Metaphos, ME605, and E601.

History
Parathion was developed by Dr. Gerhard Schrader for the German trust IG Farben in the 1940s. After the war, the Western allies seized the patent, and parathion was marketed worldwide by different companies and under different brand names. The most common German brand was E605 (banned in Germany after 2002); this was not a food-additive "E number" as used in the EU today. "E" stands for Entwicklungsnummer (German for "development number").

Handling properties
When pure, parathion is a white crystalline solid, however it is commonly distributed as a brown liquid that smells of rotting eggs or garlic. The insecticide is more or less stable, although it darkens when exposed to sunlight.

Industrial synthesis
Parathion is synthesized from diethyl dithiophosphoric acid (C2H5O)2PS2H, which is obtained by treatment of P2S5 with ethanol (methanol is used to prepare methyl Parathion). Diethyl dithiophosphoric acid is chlorinated to generate diethylthiophosphoryl chloride. The diethyl dithiophosphoric acid is then treated with sodium p-nitrophenolate.


 * 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, but indirectly. After being ingested by insects, the parathion becomes oxidized by oxidases to give paraoxon, replacing the double bonded sulfur with oxygen.


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

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

Degradation
Degradation of parathion leads to more water soluble products. Hydrolysis, which deactivates the molecule, occurs at the aryl ester bond resulting in diethylphosphate and 4-nitrophenol.
 * (C2H5O)2P(S)OC6H4NO2 + H2O → HOC6H4NO2 + (C2H5O)2P(S)OH

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

Safety
Parathion is a cholinesterase inhibitor. It generally disrupts the nervous system by inhibiting the acetylcholinesterase. It is absorbed via skin, mucous membranes, and orally. Absorbed Parathion is rapidly metabolized to Paraoxon, as described above. Paraoxon exposure can result in headaches, convulsions, poor vision, vomiting, abdominal pain, severe diarrhea, unconsciousness, tremor, dyspnea, and finally lung-edema as well as respiratory arrest. Symptoms of poisoning are known to last for extended periods of time, 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. Parathion has been used for committing suicide and deliberately poisoning other persons. It is known as "Schwiegermuttergift" (mother-in-law poison) in Germany. For this reason most formulations contain a blue dye providing warning.

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 as a UNEP Persistent Organic Pollutant and WHO Toxicity Class, "Ia, Extremely Hazardous".

Parathion is very toxic to bees, fish, birds, and other forms of wildlife. Parathion can be replaced by many safer and less toxic alternatives (less toxic organophosphates, carbamates, or synthetic pyrethroids).

Protection against poisoning
To assure human protection the end user must wear protective gloves, clothing, and a respirator of the organic-vapour. Industrial safety during the production process requires special ventilation and continuous measurement of air contamination in order not to exceed PEL levels as well as keeping personal hygiene. Frequent determination of workers' serum acetylcholinesterase activity is also helpful in regards of occupational safety, because the action of Parathion is cumulative. If an area of the body is contaminated with parathion, if possible, it should be removed immediately. Also, atropine has been used as a specific antidote.

Proposals to ban
According to the non-governmental organisation Pesticide Action Network or PAN, parathion is one of the most dangerous pesticides. This organization lists Parathion also as a 'bad actor chemical'. In the US alone more than 650 agricultural workers have been poisoned since 1966, of which 100 died. In underdeveloped countries many more people have suffered fatal and nonfatal intoxications. The World Health Organization, PAN and numerous environmental organisations propose a general and global ban. Its use is banned or restricted in 23 countries and its import is illegal in a total of 50 countries. However, a common way for pesticide manufacturers to bypass bans is to simply import ingredients from countries where it is banned, and produced the chemical in countries that have not banned it or have poor regulations. Some nations, a pesticide's use may be banned but not the manufacture and export, such as the United States. The resulting food may be imported back into the very nation where there is a ban.