Phosgene

Phosgene is the organic chemical compound with the formula COCl2. It is a colorless gas; in low concentrations, its odor resembles freshly cut hay or grass.[6] Phosgene is a valued industrial building block, especially for the production of urethanes and polycarbonate plastics. However, it is very poisonous and was used as a chemical weapon during World War I where it was responsible for 85,000 deaths. In addition to its industrial production, small amounts occur from the breakdown and the combustion of organochlorine compounds.[7]

Not to be confused with phosphine, phosphene, oxalyl chloride, or phosgene oxime.
Phosgene[1]
Full structural formula with dimensions
Space-filling model
Names
Preferred IUPAC name
Carbonyl dichloride[2]
Other names
Carbonyl chloride
CG
Carbon dichloride oxide
Carbon oxychloride
Chloroformyl chloride
Dichloroformaldehyde
Dichloromethanone
Dichloromethanal
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.000.792
EC Number
  • 200-870-3
RTECS number
  • SY5600000
UNII
UN number 1076
Properties
COCl2, also CCl2O
Molar mass 98.92 g/mol
Appearance Colorless gas
Odor Suffocating, like musty hay[3]
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)
Insoluble, reacts[4]
Solubility Soluble in benzene, toluene, acetic acid
Decomposes in alcohol and acid
Vapor pressure 1.6 atm (20°C)[3]
−48·10−6 cm3/mol
Structure
Planar, trigonal
1.17 D
Hazards
Safety data sheet ICSC 0007
T+
R-phrases (outdated) R26 R34
S-phrases (outdated) (S1/2) S9 S26 S36/37/39 S45
NFPA 704 (fire diamond)
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 code
0
4
1
Flash point Non-flammable
0.1 ppm
Lethal dose or concentration (LD, LC):
500 ppm (human, 1 min)
340 ppm (rat, 30 min)
438 ppm (mouse, 30 min)
243 ppm (rabbit, 30 min)
316 ppm (guinea pig, 30 min)
1022 ppm (dog, 20 min)
145 ppm (monkey, 1 min)[5]
3 ppm (human, 2.83 h)
30 ppm (human, 17 min)
50 ppm (mammal, 5 min)
88 ppm (human, 30 min)
46 ppm (cat, 15 min)
50 ppm (human, 5 min)
2.7 ppm (mammal, 30 min)[5]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 0.1 ppm (0.4 mg/m3)[3]
REL (Recommended)
 TWA 0.1 ppm (0.4 mg/m3) C 0.2 ppm (0.8 mg/m3) [15-minute][3]
IDLH (Immediate danger)
 2 ppm[3]
Related compounds
Related compounds
 Thiophosgene
Formaldehyde
Carbonic acid
Urea
Carbon monoxide
Chloroformic acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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

Production

Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:[7]

CO + Cl2 → COCl2 (ΔHrxn = −107.6 kJ/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(300 K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.[7]

Because of safety issues, phosgene is often produced and consumed within the same plant, and extraordinary measures are made to contain it. 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.[9] 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.[10]

Inadvertent generation

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 and with a small percentage of ethanol added. Chlorinated solvents used to remove oil from metals, such as automotive brake cleaners, are converted to phosgene by the UV rays of arc welding processes.[11]

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 through 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 possibility for people fighting fires that occur in the vicinity of refrigerant leaks from air-conditioning systems or refrigeration equipment, smoking in the vicinity of a freon refrigerant leak, or fighting fires using halon or halotron.

Phosgene can be released during building fires. In one instance, a deputy fire chief died ten days after inhaling fumes that wafted down outside a burning restaurant. After a two-day hospitalization he had appeared to recover, but ultimately suffered cardiac arrest at home following tracheobronchial inflammation, alveolar hemorrhage, and pulmonary edema. The phosgene was produced by decomposing Freon 22 after flames ducted up from a grease fire heated an air-conditioning unit on the roof and ruptured a hose.[12]

History

Phosgene was synthesized by the Cornish 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).[13] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Reactions and uses

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. The reaction of an organic substrate with phosgene is called phosgenation.

Synthesis of carbonates

Significant amounts are also used in the production of polycarbonates by its reaction with bisphenol A.[7] 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

Phosgenation of hydroxamic acids gives dioxazolone, a class of cyclic carbonate esters:[14]

RC(O)NHOH + COCl2 → RC=NOCO2 + 2 HCl

Synthesis of isocyanates

The synthesis of isocyanates from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent of "CO2+":[15]

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

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.[16] Aside from the above reactions that are widely practiced industrially, phosgene is also used to produce acyl 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 standard, a tapered thread that is known as CGA 160

Other chemistry

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.[7]

Chemical warfare
Further information: Chemical weapons in World War I and Second Italo-Ethiopian War
US Army phosgene identification poster from World War II

The collapse of international conventions against chemical weapons led to the widespread use of chlorine gas in World War I, but its lethal concentration of 0.1% was visible as a green cloud in the air, allowing troops to take readily available countermeasures. Phosgene, colorless with a more subtle "moldy hay" odor, was introduced by a group of French chemists led by Victor Grignard and first used by the French in 1915.[17] It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene.[18][19] Phosgene was more potent than chlorine, though some of the symptoms of exposure took 24 hours or more to manifest, meaning the victims were initially still capable of putting up a fight.[20]

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.[21][22][23]

In May 1928, eleven tons of phosgene escaped from a war surplus store in central Hamburg.[24] Three hundred people were poisoned, of whom 10 died.[24]

Phosgene was then only infrequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.[25] 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.[26]

Safety

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.[27]

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.[7]

Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia.[28]

Accidents
  • On January 23, 2010, an accidental release of phosgene gas at a DuPont facility in West Virginia killed one employee.[29]
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See also

References
  1. Merck Index, 11th Edition, 7310.
  2. Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 798. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
  3. NIOSH Pocket Guide to Chemical Hazards. "#0504". National Institute for Occupational Safety and Health (NIOSH).
  4. "PHOSGENE (cylinder)". Inchem (Chemical Safety Information from Intergovernmental Organizations). International Programme on Chemical Safety and the European Commission.
  5. "Phosgene". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  6. CBRNE - Lung-Damaging Agents, Phosgene May 27, 2009
  7. Wolfgang Schneider; Werner Diller. "Phosgene". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_411.
  8. 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". Journal of Molecular Spectroscopy. 83: 105–117. doi:10.1016/0022-2852(80)90314-8.
  9. Annex on Implementation and Verification ("Verification Annex") Archived 2006-05-15 at the Wayback Machine.
  10. https://itportal.decc.gov.uk/cwc_files/S2AAD_guidance.pdf.
  11. "Common Cleaners Can Turn Into Poison Gas". American Iron Magazine. TAM Communications. Archived from the original on 27 July 2009. Retrieved 14 October 2011.CS1 maint: BOT: original-url status unknown (link)
  12. Fireground Medical Operations, Albert Einstein Medical Center. "'Just a routine fire'".
  13. John Davy (1812). "On a gaseous compound of carbonic oxide and chlorine". Philosophical Transactions of the Royal Society of London. 102: 144–151. doi:10.1098/rstl.1812.0008. JSTOR 107310. 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; ... "
  14. Middleton, William J. (1983). "1,3,4-Dioxazol-2-ones: a potentially hazardous class of compounds". Journal of Organic Chemistry. 48: 3845-7. doi:10.1021/jo00169a059.
  15. R. L. Shriner, W. H. Horne, and R. F. B. Cox (1943). "p-Nitrophenyl Isocyanate". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 2, p. 453
  16. Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. doi:10.1002/047084289X.rp149
  17. Nye, Mary Jo (1999). Before big science: the pursuit of modern chemistry and physics, 1800–1940. Harvard University Press. p. 193. ISBN 0-674-06382-1.
  18. Staff (2004). "Choking Agent: CG". CBWInfo. Archived from the original on 2006-02-18. Retrieved 2007-07-30.
  19. Kiester, Edwin; et al. (2007). An Incomplete History of World War I. 1. Murdoch Books. p. 74. ISBN 1-74045-970-9.
  20. Staff (22 February 2006). "Facts About Phosgene". CDC. Archived from the original on 17 April 2003. Retrieved 2008-05-23.
  21. Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008
  22. Chemical warfare left its legacy, Lithgow Mercury, 9/09/2008
  23. Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
  24. Ryan, T.Anthony (1996). Phosgene and Related Carbonyl Halides. Elsevier. pp. 154–155. ISBN 0444824456.
  25. Yuki Tanaka, "Poison Gas, the Story Japan Would Like to Forget", Bulletin of the Atomic Scientists, October 1988, pp. 16–17
  26. Y. Yoshimi and S. Matsuno, Dokugasusen Kankei Shiryô II, Kaisetsu, Jugonen Sensô Gokuhi Shiryoshu, 1997, pp. 27–29
  27. Borak J.; Diller W. F. (2001). "Phosgene exposure: mechanisms of injury and treatment strategies". Journal of Occupational and Environmental Medicine. 43 (2): 110–9. doi:10.1097/00043764-200102000-00008. PMID 11227628.
  28. "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998.
  29. https://www.csb.gov/dupont-corporation-toxic-chemical-releases/
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