CG; carbon dichloride oxide; carbon oxychloride; Chloroformyl chloride; dichloroformaldehyde; dichloromethanone; dichloromethanal
|Molar mass||98.92 g mol−1|
|Density||4.248 g/L (15 °C, gas)
1.432 g/cm3 (0 °C, liquid)
|Melting point||−118 °C (−180 °F; 155 K)|
|Boiling point||8.3 °C (46.9 °F; 281.4 K)|
|decomposes in water|
|Solubility||soluble in benzene, toluene, acetic acid
decomposes in alcohol and acid
|Molecular shape||Planar, trigonal|
|Dipole moment||1.17 D|
|S-phrases||(S1/2) S9 S26 S36/37/39 S45|
|US health exposure limits (NIOSH):|
|TWA 0.1 ppm (0.4 mg/m3)|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles freshly cut hay or grass. In addition to its industrial production, small amounts occur naturally from the breakdown and the combustion of organochlorine compounds, such as those used in refrigeration systems. The chemical was named by combining the Greek words 'phos' (meaning light) and genesis (birth); it does not mean it contains any phosphorus (cf. phosphine).
Structure and basic properties
Phosgene is a planar molecule as predicted by VSEPR theory. The C=O distance is 1.18 Å, the C—Cl distance is 1.74 Å and the Cl—C—Cl angle is 111.8°. It is one of the simplest acid chlorides, being formally derived from carbonic acid.
- CO + Cl2 → COCl2 (ΔHrxn = −107.6kJ/mol)
The reaction is exothermic, therefore the reactor must be cooled. Typically, the reaction is conducted between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq (300K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.
Because of safety issues, phosgene is often produced and consumed within the same plant, and extraordinary measures are made to contain this toxic gas. 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. Although less dangerous than many other chemical weapons, such as sarin, phosgene is still regarded as a viable chemical warfare agent because it is so easy to manufacture when compared to the production requirements of more technically advanced chemical weapons such as the first-generation nerve agent tabun.
Upon ultraviolet (UV) radiation in the presence of oxygen, chloroform slowly converts into phosgene by a radical reaction. To suppress this photodegradation, chloroform is often stored in brown-tinted glass containers. Chlorinated compounds used to remove oil from metals, such as automotive brake cleaners, are converted to phosgene by the UV rays of arc welding processes.
Phosgene may also be produced during testing for leaks of older-style refrigerant gases. Chloromethanes (R12, R22 and others) were formerly leak-tested in situ by employing a small gas torch (propane, butane or propylene gas) with a sniffer tube and a copper reaction plate in the flame nozzle of the torch. If any refrigerant gas was leaking from a pipe or joint, the gas would be sucked into the flame via the sniffer tube and would cause a colour change of the gas flame to a bright greenish blue. In the process, phosgene gas would be created due to the thermal reaction. No valid statistics are available, but anecdotal reports suggest that numerous refrigeration technicians suffered the effects of phosgene poisoning due to their ignorance of the toxicity of phosgene, produced during such leak testing. Electronic sensing of refrigerant gases phased out the use of flame testing for leaks in the 1980s. Similarly, phosgene poisoning is a consideration for people fighting fires that are occurring in the vicinity of freon refrigeration equipment, smoking in the vicinity of a freon leak, or fighting fires using halon or halotron.
The great majority of phosgene is used in the production of isocyanates, the most important being toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). These two isocyanates are precursors to polyurethanes.
Synthesis of carbonates
Significant amounts are also used in the production of polycarbonates by its reaction with bisphenol A. Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eyeglasses. Diols react with phosgene to give either linear or cyclic carbonates (R = H, alkyl, aryl):
- HOCR2-X-CR2OH + COCl2 → 1/n [OCR2-X-CR2OC(O)-]n + 2 HCl
Synthesis of isocyanates
In the research laboratory phosgene still 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. Aside from the above reactions that are widely practiced industrially, phosgene is also used to produce acid chlorides and carbon dioxide from carboxylic acids:
- RCO2H + COCl2 → RC(O)Cl + HCl + CO2
Such acid chlorides react with amines and alcohols to give, respectively, amides and esters, which are commonly used intermediates. Thionyl chloride is more commonly and more safely employed for this application. A specific application for phosgene is the production of chloroformic esters:
- ROH + COCl2 → ROC(O)Cl + HCl
Phosgene is stored in metal cylinders. The outlet is always standared, a tapered thread that is known as CGA 160
- COCl2 + H2O → CO2 + 2 HCl
Analogously, with ammonia, one obtains urea:
- COCl2 + 4 NH3 → CO(NH2)2 + 2 NH4Cl
Phosgene was synthesized by the British chemist John Davy (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it "phosgene" in reference of the use of light to promote the reaction; from Greek, phos (light) and gene (born). It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.
Phosgene was then only frequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War. Gas weapons, such as phosgene, were produced by Unit 731 and authorized by specific orders given by Hirohito (Emperor Showa) himself, transmitted by the chief of staff of the army. For example, the Emperor authorized the use of toxic gas on 375 separate occasions during the Battle of Wuhan from August to October 1938.
Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear. The odor detection threshold for phosgene is 0.4 ppm, four times the threshold limit value. Its high toxicity arises from the action of the phosgene on the proteins in the pulmonary alveoli, the site of gas exchange: their damage disrupts the blood-air barrier, causing suffocation. It reacts with the amines of the proteins, causing crosslinking by formation of urea-like linkages, in accord with the reactions discussed above. Phosgene detection badges are worn by those at risk of exposure.
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- Annex on Implementation and Verification ("Verification Annex")
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- Davy's account of his discovery of phosgene
- International Chemical Safety Card 0007
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- U.S. CDC Emergency Preparedness & Response
- U.S. EPA Acute Exposure Guideline Levels
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- Use of Phosgene in WWII and in modern-day warfare (Refer to Section 4.C of the article)
- An experience with accidental poisoning by heated tetrachlorethylene solvent