Methyl parathion

Parathion

[par-uh-thahy-on]

}} Parathion, also called parathion-ethyl or diethyl parathion, is an organophosphate compound. Like many other organophosphate insecticides, it is a very potent insecticide and acaricide. It was originally developed by IG Farben in the 1940s. It is highly toxic to non-target organisms. Its use is banned or restricted in many countries, and there are proposals to ban it from all use.

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"), not for "European".

Properties

In its purest form, parathion consists of white crystals; however more commonly distributed forms take the form of a brown liquid which smells of rotting eggs or garlic. The insecticide is more or less stable, though it darkens when exposed to sunlight.

Industrial Synthesis

Parathion is synthesized in two steps from diethyl dithiophosphoric acid, which is obtained by treatment of P2S5 with ethanol. In the first step, diethyl dithiophosphate is chlorinated to generate diethylthiophosphoryl chloride. The diethyl dithiophosphoric acid is then treated with sodium p-nitrophenolate at elevated temperatures for extended periods of time.

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

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

Applications

As a pesticide, parathion is generally applied by spraying, and often used on 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 → (C₂H₅O)₂P(O)OC₆H₄NO₂

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 degrades and deactivates the molecule. Hydrolysis occurs at the aryl ester bond resulting in diethylphosphate and 4-nitrophenol.

(C₂H₅O)₂P(S)OC₆H₄NO₂ + H₂O → HOC₆H₄NO₂ + (C₂H₅O)₂P(S)O^- + H⁺

Under anaerobic conditions, degradation proceeds differently; the nitro group on parathion is reduced to the amine.

(C₂H₅O)₂P(S)OC₆H₄NO₂ → (C₂H₅O)₂P(S)OC₆H₄NH₂

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 in which the sulfur atom is replaced by oxygen. 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 US EPA to be a possible human carcinogen. Studies show that Parathion is toxic to fetuses, but does not cause birth defects.

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, clean protective clothing, and a respirator of the organic-vapour type when handling this material. He/she should bathe immediately after work. 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 may be 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 '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.

Parathion-methyl

A second compound of this chemical group, parathion-methyl, was also developed and marketed for similar uses. It is a distinct compound from parathion; its IUPAC name is O,O-dimethyl O-4-nitrophenyl phosphorothioate, with a molecular formula of C₈H₁₀NO₅PS. Its CAS No. is 298-00-0.

Other names for parathion-methyl include methyl parathion and dimethyl parathion; some trade names include Bladan M, Metaphos, ME605, and E601.