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Phosgene[1]
| align="center" style="background:lime;" | Full structural formula with dimensions Space-filling model
Names
IUPAC nameExpression error: Unexpected > operator.
Carbonyl dichloride
Other names
CG; carbon dichloride oxide; carbon oxychloride; Chloroformyl chloride; dichloroformaldehyde; dichloromethanone
Identifiers
CAS Number
75-44-5 YesY
ChEBI CHEBI:29365 YesY
ChemSpider 6131 YesY
EC Number 200-870-3
Jmol 3D model Interactive image
PubChem 6371
RTECS number SY5600000
UNII 117K140075 YesY
UN number 1076
Properties
Molar mass 98.92 g mol−1
Appearance colorless gas
Density 4.248 g/L (15 °C, gas)
1.432 g/cm3 (0 °C, liquid)
Melting point
Boiling point
Solubility in water
decomposes in water[2]
Solubility soluble in benzene, toluene, acetic acid
decomposes in alcohol and acid
Structure
Molecular shape
Planar, trigonal
Dipole moment
1.17 D
Hazards
EU classification (DSD)
Very toxic (T+)
R-phrases R26 R34
S-phrases (S1/2) S9 S26 S36/37/39 S45
NFPA 704
Flammability code 0: Will not burn. E.g., waterHealth code 4: Very short exposure could cause death or major residual injury. E.g., VX gasReactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g., calciumSpecial hazards (white): no codeNFPA 704 four-colored diamond
0
4
1
Flash point
Threshold Limit Value
0.1 ppm
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
YesY verify (what is YesYN ?)
Infobox references

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.[3] 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.[4] 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[edit | edit source]

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°.[5] It is one of the simplest acid chlorides, being formally derived from carbonic acid.

Production[edit | edit source]

Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:[4]

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.[4]

Because of safety issues, phosgene is almost always 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.[6] 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.[7]

Adventitious occurrence[edit | edit source]

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.[8]

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.[citation needed] 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.

Uses[edit | edit source]

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 isocyanates are precursors to polyurethanes.

Synthesis of carbonates[edit | edit source]

Significant amounts are also used in the production of polycarbonates by its reaction with bisphenol A.[4] Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eye glasses. 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[edit | edit source]

The synthesis of isocyanates from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent of "CO2+":[9]

RNH2 + COCl2 → RN=C=O + 2 HCl (R = alkyl, aryl)

Such reactions are conducted in the presence of a base such as pyridine that absorbs the hydrogen chloride.

Laboratory uses[edit | edit source]

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.[10] 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

Other chemistry[edit | edit source]

Although it is somewhat hydrophobic, phosgene reacts with water to release hydrogen chloride and carbon dioxide:

COCl2 + H2O → CO2 + 2 HCl

Analogously, with ammonia, one obtains urea:

COCl2 + 4 NH3 → CO(NH2)2 + 2 NH4Cl

Halide exchange with nitrogen trifluoride and aluminium tribromide gives COF2 and COBr2, respectively.[4]

History[edit | edit source]

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).[11] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Chemical warfare[edit | edit source]

Following the extensive use of phosgene gas in combat during World War I, it was stockpiled by various countries as part of their secret chemical weapons programs.[12][13][14]

In May 1928, eleven tons of phosgene escaped from a war surplus store in central Hamburg.[15] 300 people were poisoned of whom 10 died.[15]

US Army phosgene identification poster from World War II

Phosgene was then only frequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.[16] 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.[17]

Safety[edit | edit source]

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.[18] 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.[4]

Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia.[19]

See also[edit | edit source]

References[edit | edit source]

  1. Merck Index, 11th Edition, 7310.
  2. http://www.inchem.org/documents/icsc/icsc/eics0007.htm
  3. CBRNE - Lung-Damaging Agents, Phosgene May 27, 2009
  4. 4.0 4.1 4.2 4.3 4.4 4.5 "Ullmann's Encyclopedia of Industrial Chemistry". Phosgene (cylinder). Weinheim: Wiley-VCH. 2005. Digital object identifier:10.1002/14356007.a19_411. http://www.inchem.org/documents/icsc/icsc/eics0007.htm. 
  5. Nakata, M.; Kohata, K.; Fukuyama, T.; Kuchitsu, K. (1980). "Molecular Structure of Phosgene as Studied by Gas Electron Diffraction and Microwave Spectroscopy. The rz Structure and Isotope Effect". pp. 105–117. Digital object identifier:10.1016/0022-2852(80)90314-8. 
  6. Annex on Implementation and Verification ("Verification Annex")
  7. https://itportal.decc.gov.uk/cwc_files/S2AAD_guidance.pdf
  8. "Common Cleaners Can Turn Into Poison Gas". American Iron Magazine. TAM Communications. http://www.brewracingframes.com/id75.htm. Retrieved 14 October 2011. 
  9. "p-Nitrophenyl Isocyanate". 1943. http://www.orgsyn.org/demo.aspx?prep=CV2P0453. ; "Coll. Vol.". pp. 453. 
  10. Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. doi: 10.1002/047084289X.rp149
  11. John Davy (1812). "On a Gaseous Compound of Carbonic Oxide and Chlorine". pp. 144–151. Digital object identifier:10.1098/rstl.1812.0008. JSTOR 107310. 
  12. Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008
  13. Chemical warfare left its legacy, Lithgow Mercury, 9/09/2008
  14. Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
  15. 15.0 15.1 Ryan, T.Anthony (1996). Phosgene and Related Carbonyl Halides. Elsevier. pp. 154–155. ISBN 0444824456. 
  16. Yuki Tanaka, "Poison Gas, the Story Japan Would Like to Forget", Bulletin of the Atomic Scientists, October 1988, p. 16–17
  17. Y. Yoshimi and S. Matsuno, Dokugasusen Kankei Shiryô II, Kaisetsu, Jugonen Sensô Gokuhi Shiryoshu, 1997, p. 27–29
  18. Borak J., Diller W. F. (2001). "Phosgene exposure: mechanisms of injury and treatment strategies". pp. 110–9. Digital object identifier:10.1097/00043764-200102000-00008. PMID 11227628. 
  19. "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998. http://www.inchem.org/documents/hsg/hsg/hsg106.htm. 

External links[edit | edit source]


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