Phosgene

Phosgene is the organic chemical compound with the formula COCl2. It is a colorless gas; in low concentrations, its odor resembles that of freshly cut hay or grass.[6] Phosgene is a valued industrial building block, especially for the production of precursors of polyurethanes and polycarbonate plastics.

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R-phrases (outdated) R26 R34
S-phrases (outdated) (S1/2) S9 S26 S36/37/39 S45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).
Y verify (what is YN ?)
Infobox references

It is very poisonous and was used as a chemical weapon during World War I, when 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]

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. 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]

As a scheduled chemical weapon, phosgene is transported under highly regulated conditions. Phosgene is typically produced and consumed within the same plant, and extraordinary measures are made to contain it. Manufacture is typically an 'on demand' process where phosgene is produced and consumed at equivalent rates, this means that the amount of phosgene in the system at any time is fairly small compared to production rate.

Inadvertent generation

Upon ultraviolet (UV) radiation in the presence of oxygen, chloroform slowly converts into phosgene.

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).[9] 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.[7]

Synthesis of carbonates.

 
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
One example is the reaction of phosgene with bisphenol A.[7] to form Polycarbonates.

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+":[10]

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

Such reactions are, on laboratory scale, conducted in the presence of a base such as pyridine that absorbs the hydrogen chloride. On industrial scale, phosgene is used in excess to increase yield and avoid side reactions. The excessive Phosgene is separated during the work-up of resulting end products and recycled to the process, remaining amounts of phosgene being decomposed with water, using activated carbon as a catalyst.

Laboratory uses

In the research laboratory phosgene 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.[11] 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 bulks and metal cylinders. The outlet of cylinders is always standard, a tapered thread that is known as CGA 160

Industrial use

Phosgene is used in industry preferably for the production of "aromatic" Diisocyanates like Toluenediisocyanate (TDI) and Methlyenediphenyldiisocyanate (MDI ), precursors for production of polyurethanes, and for the production of polycarbonate (PC), which is used for the production of advanced plastics like compact disks or eye lenses. More than 90 % of the worldwide produced phosgene is being produced for these uses in a dimension of more than 3 Mio tons per year, the biggest production units are located in United Arabic Republic, USA (Tx and La), Germany, China (Shanghai area), Japan and South Korea. The most important producers are DOW, Covestro (Bayer outsource), and BASF. Another use is for production of "aliphatic" diisocyanates like Hexamethylenediisocyanate (HDI) and Isophoronediisocyanate (IPDI), precursors for the production of advanced coatings. One more important use is the production of monoisocanates like Methylisocyanate, precursors for the production of herbicides.

Other chemistry

Phosgene reacts with water to release hydrogen chloride and carbon dioxide:

COCl2 + H2O → CO2 + 2 HCl

Analogously, upon contact with ammonia, it converts to 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

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.[12] Although less toxic than many other chemical weapons 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 the first-generation nerve agent tabun.[13]

Phosgene was first deployed as a chemical weapon by the French in 1915 in World War I.[14] It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene.[15][16] 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.[17][18][19]

Phosgene was then only infrequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.[20] Gas weapons, such as phosgene, were produced by Unit 731

Toxicology and Safety

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.[21]

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 OH-, NH2- and OH- groups of the proteins in the pulmonary alveoli, the site of gas exchange, forming ester-, amide-, and thioester functional groups in accord with the reactions discussed above. They disrupt the blood–air barrier, causing a lung edema. The extent of damage in the alveoli depends not primarily on the concentration of phosgene in the inhaled air, but on the dose (amount of inhaled phosgene, approximately "concentration" X "durance of exposition".[22][23] Therefore persons who may be exposed to phosgene in case of accidental releases usually wear indicator badges closely to nose and mouth, indicating the inhaled dose.[24] This allows an immediate adequate therapy. In case of low or moderate quantities of inhaled phosgene the exposed person would be observed and be subjected to precautionary therapy and then released after several hours. In case of doses of inhaled phosgene above 150 ppm x min a pulmonary edema often develops, which can detected by X-ray and regressive oxygen concentration in the blood. Several hours after exposition, in some cases 2 to 3 days, inhalation can eventually result in a fatality. The risk connected to a phosgene inhalation is based not so much on its toxicity (modern chemical weapons like sarin or tabun are more toxic by far) but on its typical effects: the affected person may not develop any symptoms for hours, until finally an edema would develop and then be treated medically, and therapy could be too late.[25] All fatalities that occured connected to accidental releases with the industrial handling of phosgene went this way. On the other hand pulmonary edemas , once survived, use to heal out mid - and longterm without consequences, only for some days or weeks lung function might be limited.[26]

Accidental release of phosgene may be mitigated with ammonia, in case of liquid spills (phosgene and phosgene solutions) absorbent and sodium carbonate may be applied .[27]

Accidents
  • The first major incident happened in May 1928, eleven tons of phosgene escaped from a war surplus store in central Hamburg.[28] Three hundred people were poisoned, of whom ten died.[28]
  • In the second half of 20th century several fatal phosgene-related incidents happened in Europe, Asia and the US, most of them investigated by authorities and the outcome having been made accessible to the public. Phosgene was initially blamed for the Bhopal disaster, but investigations clearly resulted in methylisocanate being responsible for the numerous intoxications and fatalities.
  • The last major incidents happened on January 23, 2010 and in May 2016: An accidental release of phosgene gas at a DuPont facility in West Virginia killed one employee.[29] The US Chemical Safety Board released a video detailing the accident. On May 26, phosgene was released in a BASF plant in South Korea, a contractor inhaled a lethal dose of phosgene.[30]

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. 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; ... "
  10. 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
  11. Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. doi:10.1002/047084289X.rp149
  12. Annex on Implementation and Verification ("Verification Annex") Archived 2006-05-15 at the Wayback Machine.
  13. https://itportal.decc.gov.uk/cwc_files/S2AAD_guidance.pdf.
  14. 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.
  15. Staff (2004). "Choking Agent: CG". CBWInfo. Archived from the original on 2006-02-18. Retrieved 2007-07-30.
  16. Kiester, Edwin; et al. (2007). An Incomplete History of World War I. 1. Murdoch Books. p. 74. ISBN 978-1-74045-970-9.
  17. Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008
  18. Chemical warfare left its legacy, Lithgow Mercury, 9/09/2008
  19. Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
  20. Yuki Tanaka, "Poison Gas, the Story Japan Would Like to Forget", Bulletin of the Atomic Scientists, October 1988, pp. 16–17
  21. 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. S2CID 41169682.
  22. Werner F. Diller, Early Diagnosis of Phosgene Overexposure.Toxicology and Industrial Health, Vol.1, Nr.2, April 1985, p. 73 -80
  23. W. F. Diller, R. Zante : Zentralbl. Arbeitsmed. Arbeitsschutz Prophyl. Ergon. 32, (1982) 60 -368
  24. 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
  25. 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
  26. W.F. Diller, F. Schnellbächer, F. Wüstefeld : Zentralbl. Arbeitsmed. Arbeitsschutz Prophyl. 29 (1979) p.5-16
  27. "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998.
  28. Ryan, T.Anthony (1996). Phosgene and Related Carbonyl Halides. Elsevier. pp. 154–155. ISBN 0444824456.
  29. https://www.csb.gov/dupont-corporation-toxic-chemical-releases/
  30. https://www.youtube.com/watch?v=ISNGimMXL7M
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