Phosgene: Wikis

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Phosgene[1]
Phosgene-dimensions-2D.svg
Phosgene-3D-vdW.png
IUPAC name
Other names CG; carbon dichloride oxide; carbon oxychloride; carbonyl dichloride; chloroformyl chloride; dichloroformaldehyde; dichloromethanone
Identifiers
CAS number 75-44-5 Yes check.svgY
PubChem 6371
EC number 200-870-3
UN number 1076
RTECS number SY5600000
SMILES
Properties
Molecular formula CCl2O
Molar mass 98.92 g mol-1
Appearance colorless gas
Density 4.248 g/cm3 (15 °C)
1.432 g/cm3 (0 °C)
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
NFPA 704.svg
0
4
1
 
Flash point non-flammable
Related compounds
Related compounds Thiophosgene
Formaldehyde
Carbonic acid
Urea
Carbon monoxide
Chloroformic acid
 Yes check.svgY (what is this?)  (verify)
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, and is also a valued industrial reagent and building block in organic synthesis. In low concentrations, its odor resembles freshly cut hay or grass. Some soldiers during the First World War stated that it smelled faintly of May Blossom. In addition to its industrial production, small amounts occur naturally from the breakdown of chlorinated compounds and the combustion of chlorine-containing organic compounds.[2]

Contents

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°.[3] 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. The equation is described as follows:[2]

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 because above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq (300K) = 0.05M. 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.[4] Although much less dangerous than most other chemical weapons (e.g. mustard gas), 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.

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Adventitious occurrence

Upon ultraviolet 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 oils from metals may also react under the UV created during arc welding to produce phosgene.

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.[2] 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.[5] 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): [6]

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.[2]

History

Phosgene was synthesized by the chemist John Davy (1790-1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it in reference to use of light to promote the reaction; from Greek, phos (light) and gene (born).[7] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Chemical warfare

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.[8][9][10].

Phosgene was then only frequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War. [11] Gas weapons, such as phosgene, were produced by unit 731 and authorized by specific orders given by 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.[12]

Safety

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.[13] Phosgene can be detected at 0.4 ppm, which is four times the Threshold Limit Value. Its high toxicity arises, not from hydrogen chloride released by hydrolysis, but by the action of the phosgene on the proteins in the pulmonary alveoli. The alveoli are the site of gas exchange, and their damage disrupts the blood-air barrier, causing suffocation. Phosgene detection badges are worn by those at risk of exposure.[2]

Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia.[14]

References

  1. ^ Merck Index, 11th Edition, 7310.
  2. ^ 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. Article Online Posting Date: June 15, 2000
  3. ^ 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.  
  4. ^ Annex on Implementation and Verification ("Verification Annex")
  5. ^ Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. doi: 10.1002/047084289X.rp149
  6. ^ 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  
  7. ^ 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. http://links.jstor.org/sici?sici=0261-0523%281812%29102%3C144%3AOAGCOC%3E2.0.CO%3B2-1.  
  8. ^ Base's phantom war reveals its secrets, Lithgow Mercury, 7/08/2008
  9. ^ Chemical warfare left its legacy, Lithgow Mercury, 9/09/2008
  10. ^ Chemical bombs sit metres from Lithgow families for 60 years, The Daily Telegraph, September 22, 2008
  11. ^ Yuki Tanaka, Poison Gas, the Story Japan Would Like to Forget, Bulletin of the Atomic Scientists, October 1988, p. 16-17
  12. ^ Y. Yoshimi and S. Matsuno, Dokugasusen Kankei Shiryô II, Kaisetsu, Jugonen Sensô Gokuhi Shiryoshu, 1997, p.27-29
  13. ^ 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.  
  14. ^ "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998. http://www.inchem.org/documents/hsg/hsg/hsg106.htm.  

External links


Simple English

Not to be confused with phosgene oxime.

[[File:|200px|thumb|A phosgene tank.]] [[File:|100px|thumb|Chemical note]] Phosgene is the chemical compound with the formula COCl2. This gas has no color. The gas was used as a weapon during World War I. It is responsible for most of the deaths related to poison gas during the war. Today, phosgene is used as a tool. It is used for organic synthesis. When there is a small amount in the air, the smell is similar to that of freshly cut hay or grass. Some soldiers during the First World War have stated that it smelled a little like May Blossom. When chlorine is burned, small amounts of phosgene are given off. They are given off from the breakdown of chlorinated compounds.[1]

Contents

Uses

Today, most phosgene is used to produce isocyanates. The most important isocanates in this context are toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). Both are precursors to polyurethanes. Large amounts are also used to produce polycarbonates. Polycarbonate is produced from its reaction with bisphenol A.[1] Polycarbonates are an important class of engineering thermoplastic. They can be found in lenses in eye glasses.

Safety

Phosgene is a strong poison. Its smell may not be noticed. Signs of exposure may sometimes be detected slowly.[2] Phosgene can be seen at 0.4 parts per million. This amount is four times the safe amount. Its high ability to poison doesn't come from the hydrogen chloride released by a process. It comes from the action of the phosgene on the proteins in the pulmonary sac of the lungs. The sacs are where the oxygen goes into the body. Their damage makes it hard for the blood and air move oxygen and carbon. This will make breathing very hard. Badges are worn by those at risk of exposure.[1]

Sodium bicarbonate may be used to make spills of phosgene harmless. Gas spills may be made harmless with a special liquid spray.[3]

Other websites

References

  1. 1.0 1.1 1.2 Wolfgang Schneider and Werner Diller "Phosgene" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2002. DOI: 10.1002/14356007.a19_411. Article Online Posting Date: June 15, 2000
  2. Borak J., Diller W. F. (2001). [Expression error: Unexpected < operator "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. 
  3. "Phosgene: Health and Safety Guide". International Programme on Chemical Safety. 1998. http://www.inchem.org/documents/hsg/hsg/hsg106.htm. 


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