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Phosgene is an organic chemical compound with the chemical formula . It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. CBRNE - Lung-Damaging Agents, Phosgene May 27, 2009 It can be thought of chemically as the double acyl chloride analog of carbonic acid, or structurally as formaldehyde with the hydrogen atoms replaced by chlorine atoms. In 2013, about 75–80 % of global phosgene was consumed for , 18% for and about 5% for other fine chemicals. (2025). 9789279765896, EU Publications Office. ISBN 9789279765896
Phosgene is extremely poisonous and was used as a chemical weapon during World War I, where it was responsible for 85,000 deaths. It is a highly potent pulmonary irritant and quickly filled enemy trenches due to it being a heavy gas.
It is classified as a Schedule 3 substance under the Chemical Weapons Convention. In addition to its industrial production, small amounts occur from the breakdown and the combustion of Organochloride, such as chloroform.
This reaction is exothermic and is typically performed between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq(300 K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.
Phosgene is fairly simple to produce, but is listed as a Schedule 3 substance under the Chemical Weapons Convention. As such, it is usually considered too dangerous to transport in Bulk cargo. Instead, phosgene is usually produced and consumed within the same plant, as part of an "on demand" process. This involves maintaining equivalent rates of production and consumption, which keeps the amount of phosgene in the system at any one time fairly low, reducing the risks in the event of an accident. Some batch production does still take place, but efforts are made to reduce the amount of phosgene stored.
Phosgene in the troposphere can last up to about 70 days and is removed primarily by hydrolysis with ambient humidity or cloudwater. Less than 1% makes it to the stratosphere, where it is expected to have a lifetime of several years, since this layer is much drier and phosgene decomposes slowly through UV photolysis. It plays a minor part in ozone depletion.
Aside from the widely used reactions described above, phosgene is also used to produce from :
Phosgene is used to produce such as benzyl chloroformate:
With , phosgene (or its trimer) reacts to give amino acid N-carboxyanhydrides. More generally, phosgene acts to link two nucleophiles by a carbonyl group. For this purpose, alternatives to phosgene such as carbonyldiimidazole (CDI) are safer, albeit expensive. CDI itself is prepared by reacting phosgene with imidazole.
Phosgene is stored in Gas cylinder. In the US, the cylinder valve outlet is a tapered thread known as "CGA 160" that is used only for phosgene.
Analogously, upon contact with ammonia, it converts to urea:
Halide exchange with nitrogen trifluoride and aluminium tribromide gives and carbonyl bromide, respectively.
Phosgene was first deployed as a chemical weapon by the French in 1915 in World War I. It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene. Phosgene was more potent than chlorine, though some symptoms took 24 hours or more to manifest.
Following the extensive use of phosgene during World War I, it was stockpiled by various countries. Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008 Chemical warfare left its legacy , Lithgow Mercury, 9/09/2008 Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
Phosgene was then only infrequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.Yuki Tanaka, "Poison Gas, the Story Japan Would Like to Forget", Bulletin of the Atomic Scientists, October 1988, pp. 16–17 Gas weapons, such as phosgene, were produced by the IJA's Unit 731.
At low concentrations, phosgene may have a pleasant odor of freshly mown hay or green corn, but has also been described as sweet, like rotten banana peels.
The odor detection threshold for phosgene is 0.4 ppm, four times the threshold limit value (time weighted average). Its high toxicity arises from the action of the phosgene on the , and groups of the in pulmonary alveoli (the site of gas exchange), respectively forming ester, amide and thioester functional groups in accord with the reactions discussed above. This results in disruption of the blood–air barrier, eventually causing pulmonary edema. The extent of damage in the alveoli does not primarily depend on phosgene concentration in the inhaled air, with the dose (amount of inhaled phosgene) being the critical factor. Dose can be approximately calculated as "concentration" × "duration of exposure".Werner F. Diller, Early Diagnosis of Phosgene Overexposure. Toxicology and Industrial Health, Vol.1, Nr.2, April 1985, p. 73 -80W. F. Diller, R. Zante : Zentralbl. Arbeitsmed. Arbeitsschutz Prophyl. Ergon. 32, (1982) 60 -368 Therefore, persons in workplaces where there exists risk of accidental phosgene release usually wear indicator badges close to the nose and mouth. Such badges indicate the approximate inhaled dose, which allows for immediate treatment if the monitored dose rises above safe limits.W. F.Diller, E.Drope, E. Reichold: Ber. Int. Kolloq. Verhütung von Arbeitsunfällen und Berufskrankheiten Chem. Ind.6 th (1979) Chem. Abstr. 92 (1980) 168366x
In case of low or moderate quantities of inhaled phosgene, the exposed person is to be monitored and subjected to precautionary therapy, then released after several hours. For higher doses of inhaled phosgene (above 150 ppm × min) a pulmonary edema often develops which can be detected by Radiography and regressive blood oxygen concentration. Inhalation of such high doses can eventually result in fatality within hours up to 2–3 days of the exposure.
The risk connected to a phosgene inhalation is based not so much on its toxicity (which is much lower in comparison to modern chemical weapons like sarin or tabun) but rather on its typical effects: the affected person may not develop any symptoms for hours until an edema appears, at which point it could be too late for medical treatment to assist.W. F. Diller: Radiologische Untersuchungen zur verbesserten Frühdiagnose von industriellen Inhalationsvergiftungen mit verzögertem Wirkungseintritt, Verlag für Medizin Dr. E. Fischer, Heidelberg. Zentralbatt für Arbeitsmedizin, Arbeitsschutz und Ergonomie, Nr. 3, Mai 2013, p. 160 - 163 Nearly all fatalities as a result of accidental releases from the industrial handling of phosgene occurred in this fashion. On the other hand, pulmonary edemas treated in a timely manner usually heal in the mid- and longterm, without major consequences once some days or weeks after exposure have passed.W.F. Diller, F. Schnellbächer, F. Wüstefeld : Zentralbl. Arbeitsmed. Arbeitsschutz Prophyl. 29 (1979) p.5-16Results From the US Industry-Wide Phosgene Surveillance "The Diller Registry" : Journal of Occ. and Env. Med., March 2011-Vol.53-iss. 3 p.239- 244 Nonetheless, the detrimental health effects on pulmonary function from untreated, chronic low-level exposure to phosgene should not be ignored; although not exposed to concentrations high enough to immediately cause an edema, many synthetic chemists ( e.g. Leonidas Zervas) working with the compound were reported to experience chronic respiratory health issues and eventual respiratory failure from continuous low-level exposure.
If accidental release of phosgene occurs in an industrial or laboratory setting, it can be mitigated with ammonia gas; in the case of liquid spills ( e.g. of diphosgene or phosgene solutions) an absorbent and sodium carbonate can be applied.
Inadvertent generation
Atmospheric chemistry
Simple slowly convert into phosgene when exposed to ultraviolet (UV) irradiation in the presence of oxygen. Before the discovery of the ozone hole in the late 1970s large quantities of organochlorides were routinely used by industry, which inevitably led to them entering the atmosphere. In the 1970-80s phosgene levels in the troposphere were around 20-30 parts per trillion by volume (peak 60 parts per trillion by volume). These levels had not decreased significantly nearly 30 years later, despite organochloride production becoming restricted under the Montreal Protocol.
Combustion
Carbon tetrachloride () can turn into phosgene when exposed to heat in air. This was a problem as carbon tetrachloride is an effective fire suppressant and was formerly in widespread use in fire extinguishers. There are reports of fatalities caused by its use to fight fires in . Carbon tetrachloride's generation of phosgene and its own toxicity mean it is no longer used for this purpose.
Biologically
Phosgene is also formed as a metabolite of chloroform, likely via the action of cytochrome P-450.
History
Phosgene was synthesized by the Cornish people chemist John Davy (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it "phosgene" from Greek language φῶς (phos, light) and γεννάω (gennaō, to give birth) in reference of the use of light to promote the reaction. Phosgene was named on p. 151: " ... it will be necessary to designate it by some simple name. I venture to propose that of phosgene, or phosgene gas; from φως, light, γινομαι, to produce, which signifies formed by light; ... " It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.
Reactions and uses
The reaction of an organic substrate with phosgene is called phosgenation. Phosgenation of give carbonates (R = Hydrogen, alkyl, aryl), which can be either linear or cyclic:
An example is the reaction of phosgene with bisphenol A to form . Phosgenation of diamines gives di-isocyanates, like toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI). In these conversions, phosgene is used in excess to increase yield and minimize side reactions. The phosgene excess is separated during the work-up of resulting end products and recycled into the process, with any remaining phosgene decomposed in water using activated carbon as the catalyst. Diisocyanates are precursors to polyurethanes. More than 90% of the phosgene is used in these processes, with the biggest production units located in the United States (Texas and Louisiana), Germany, Shanghai, Japan, and South Korea. The most important producers are Dow Chemical, Covestro, and BASF. Phosgene is also used to produce monoisocyanates, used as pesticide precursors ( e.g. methyl isocyanate (MIC).
For this application, thionyl chloride is commonly used instead of phosgene.
Laboratory uses
The synthesis of from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent synthon "CO2+":
Such reactions are conducted on laboratory scale in the presence of a base such as pyridine that neutralizes the hydrogen chloride side-product.
In these syntheses, phosgene is used in excess to prevent formation of the corresponding carbonate ester.
Alternatives to phosgene
In the research laboratory, due to safety concerns phosgene nowadays finds limited use in organic synthesis. A variety of substitutes have been developed, notably trichloromethyl chloroformate ("diphosgene"), a liquid at room temperature, and bis(trichloromethyl) carbonate ("triphosgene"), a crystalline substance.Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York.
Other reactions
Phosgene reacts with water to release hydrogen chloride and carbon dioxide:
Chemical warfare
It is listed on Schedule 3 of the Chemical Weapons Convention: All production sites manufacturing more than 30 tonnes per year must be declared to the OPCW. Annex on Implementation and Verification ("Verification Annex") . Although less toxic than many other such as sarin, phosgene is still regarded as a viable chemical warfare agent because of its simpler manufacturing requirements when compared to that of more technically advanced chemical weapons such as tabun, a first-generation
.
Toxicology and safety
Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.
Accidents
See also
External links
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