Phosgene

Not to be confused with phosphine, oxalyl chloride, or phosgene oxime.

Phosgene[1]
IUPAC name Carbonyl dichloride
Other names CG; carbon dichloride oxide; carbon oxychloride; chloroformyl chloride; dichloroformaldehyde; dichloromethanone
Identifiers
CAS number	75-44-5 Y
PubChem	6371
ChemSpider	6131 Y
UNII	117K140075 Y
EC number	200-870-3
UN number	1076
ChEBI	CHEBI:29365 Y
RTECS number	SY5600000
Jmol-3D images	Image 1
SMILES ClC(Cl)=O
InChI InChI=1S/CCl2O/c2-1(3)4 Y Key: YGYAWVDWMABLBF-UHFFFAOYSA-N Y InChI=1/CCl2O/c2-1(3)4 Key: YGYAWVDWMABLBF-UHFFFAOYAH
Properties
Molecular formula	CCl2O
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	−118 °C, 155 K, -180 °F
Boiling point	8.3 °C, 281 K, 47 °F
Solubility in water	hydrolysis
Solubility	soluble in benzene, toluene, acetic acid decomposes in alcohol and acid
Structure
Molecular shape	Planar, trigonal
Dipole moment	1.17 D
Hazards
MSDS	ICSC 0007
EU Index	006-002-00-8
EU classification	Very toxic (T+)
R-phrases	R26 R34
S-phrases	(S1/2) S9 S26 S36/37/39 S45
NFPA 704	0 4 1
Flash point	non-flammable
Threshold Limit Value	0.1 ppm
Related compounds
Related compounds	Thiophosgene Formaldehyde Carbonic acid Urea Carbon monoxide Chloroformic acid
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.^[2] 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.^[3] The name, sounding similar to "phosphine", does not mean it has any phosphorus. The chemical was named by combining the Greek words ‘phos’ (meaning light) and genesis (birth).

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°.^[4] It is one of the simplest acid 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:^[3]

CO + Cl₂ → COCl₂ (ΔH_rxn = −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, K_eq (300K) = 0.05. Approximately 5000 tonnes were produced in 1989.

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.^[5] 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.

Adventitious occurrence

Upon ultraviolet (UV) radiation in the presence of oxygen, chloroform slowly converts into phosgene via 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.^[6]

Phosgene may also be produced during testing for leaks of older-style refrigerant gasses. 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.

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 isocyanates are precursors to polyurethanes. Significant amounts are also used in the production of polycarbonates via its reaction with bisphenol A.^[3] Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eye glasses.

Organic synthesis

Although phosgene still finds use in organic synthesis, a variety of substitutes have been developed, notably trichloromethyl chloroformate (“diphosgene”), which is a liquid at room temperature, and bis(trichloromethyl) carbonate (“triphosgene”), a crystalline substance.^[7] The following are the three most useful reactions involving phosgene.

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

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⁺" (R = alkyl, aryl): ^[8]

RNH₂ + COCl₂ → RN=C=O + 2 HCl

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

Synthesis of acid chlorides

It 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

Inorganic 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.^[3]

History

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).^[9] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Chemical warfare

Further information: Use of poison gas in World War I and Second Italo-Abyssinian War

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.^[10][11][12]

Phosgene was then only frequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.^[13] 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.^[14]

Safety

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.^[15] Phosgene can be detected at 0.4 ppm, which is four times the Threshold Limit Value. Its high toxicity arises from the action of the phosgene on the proteins in the pulmonary alveoli, which are the site of gas exchange: their damage disrupts the blood-air barrier, causing suffocation. It reacts with the amines of the proteins, causing crosslinking via formation of urea-like linkages, in accord with the reactions discussed above. Phosgene detection badges are worn by those at risk of exposure.^[3]

Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia.^[16]

References

1. ^ Merck Index, 11th Edition, 7310.
2. ^ CBRNE - Lung-Damaging Agents, Phosgene May 27, 2009
3. ^ ^{a b c d e} Wolfgang Schneider and Werner Diller "Phosgene" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2002. doi: 10.1002/14356007.a19_411.
4. ^ Nakata, M.; Kohata, K.; Fukuyama, T.; Kuchitsu, K. (1980). "Molecular Structure of Phosgene as Studied by Gas Electron Diffraction and Microwave Spectroscopy. The r_z Structure and Isotope Effect". Journal of Molecular Spectroscopy 83: 105–117. doi:10.1016/0022-2852(80)90314-8. 
5. ^ Annex on Implementation and Verification ("Verification Annex")
6. ^ "Common Cleaners Can Turn Into Poison Gas". American Iron Magazine. TAM Communications. http://www.brewracingframes.com/id75.htm. Retrieved 14 October 2011. 
7. ^ Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. doi: 10.1002/047084289X.rp149
8. ^ R. L. Shriner, W. H. Horne, and R. F. B. Cox (1943), "p-Nitrophenyl Isocyanate", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV2P0453 ; Coll. Vol. 2: 453 
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. 
10. ^ Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008
11. ^ Chemical warfare left its legacy, Lithgow Mercury, 9/09/2008
12. ^ Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
13. ^ Yuki Tanaka, Poison Gas, the Story Japan Would Like to Forget, Bulletin of the Atomic Scientists, October 1988, p. 16-17
14. ^ Y. Yoshimi and S. Matsuno, Dokugasusen Kankei Shiryô II, Kaisetsu, Jugonen Sensô Gokuhi Shiryoshu, 1997, p.27-29
15. ^ 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. 
16. ^ "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998. http://www.inchem.org/documents/hsg/hsg/hsg106.htm. 

See also

• Diphosgene
• Triphosgene

External links

• Davy's account of his discovery of phosgene
• International Chemical Safety Card 0007
• NIOSH Pocket Guide to Chemical Hazards
• U.S. CDC Emergency Preparedness & Response
• U.S. EPA Acute Exposure Guideline Levels
• Regime For Schedule 3 Chemicals And Facilities Related To Such Chemicals, OPCW website
• CBWInfo website
• 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