Phosgene
47107
223643519
2008-07-05T01:25:15Z
Lightbot
7178666
Units/dates/other
{{Chembox new
| Name = Phosgene
| ImageFile = Phosgene-dimensions-2D.png
| ImageFile1 = Phosgene-3D-vdW.png
| IUPACName = Dichloromethanal<br />Carbonyl chloride
| OtherNames = Phosgene; CG; carbonic acid dichloride; carbon dichloride oxide; carbon oxychloride; carbonyl dichloride; chloroformyl chloride; dichloroformaldehyde; dichloromethanone
| Section1 = {{Chembox Identifiers
| SMILES = O=C(Cl)Cl
| CASNo = 75-44-5
| RTECS = SY5600000
}}
| Section2 = {{Chembox Properties
| Formula = CCl<sub>2</sub>O
| MolarMass = 98.9 g mol<sup>-1</sup>
| Appearance = colorless gas
| Density = 4.248 g dm<sup>-3</sup>, gas (15 °C)
| Solubility = hydrolysis
| MeltingPt = −118 °C (155 K)
| BoilingPt = 8 °C (281 K)
}}
| Section3 = {{Chembox Structure
| MolShape = Planar, trigonal
| Dipole = 1.17 [[Debye|D]]
}}
| Section7 = {{Chembox Hazards
| ExternalMSDS = http://www.vngas.com/pdf/g67.pdf
| NFPA-H = 4
| NFPA-F = 0
| NFPA-R = 1
| FlashPt = non-flammable
}}
| Section8 = {{Chembox Related
| OtherCpds = [[Carbonic acid]]; [[urea]]; [[carbon monoxide]]; [[chloroformic acid]]
}}
}}
'''Phosgene''' is the [[chemical compound]] with the [[chemical formula|formula]] Cl<sub>2</sub>CO. This colourless gas gained infamy as a [[chemical weapon]] during [[World War I]], but it is also a valued industrial reagent and building block in [[organic synthesis]]. In low concentrations, its odor resembles freshly cut hay or grass. In addition to its industrial production, small amounts occur naturally from the breakdown of chlorinated compounds and the [[combustion]] of [[chlorine]]-containing [[organic chemistry|organic]] compounds. Approximately 5000 Mg were produced in 1989.<ref name=Ullmann>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</ref>
==Structure and basic properties==
Phosgene is a planar molecule as predicted by [[VSEPR theory]]. The C=O distance is 1.18 [[angstrom (unit)|Å]], the C---Cl distance is 1.74 Å and the Cl---C---Cl angle is 111.8°.<ref>{{cite journal | author = Nakata, M.; Kohata, K.; Fukuyama, T.; Kuchitsu, K. | title = Molecular Structure of Phosgene as Studied by Gas Electron Diffraction and Microwave Spectroscopy. The r<sub>z</sub> Structure and Isotope Effect | journal = [[Journal of Molecular Spectroscopy]] | year = 1980 | volume = 83 | pages = 105–117 | doi=10.1016/0022-2852(80)90314-8}}</ref> It is one of the simplest [[acid chloride]]s, 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:<ref name=Ullmann/>
:CO + Cl<sub>2</sub> → COCl<sub>2</sub> ΔH<sub>rxn</sub> = −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 decomposes back into carbon monoxide and chlorine, K<sub>eq</sub> (300K) = 0.05M.
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 [[List of Schedule 3 substances (CWC)|schedule 3]] of the [[Chemical Weapons Convention]]: all production sites manufacturing more than 30 tonnes per year must be declared to the [[OPCW]].<ref>[http://www.opcw.org/html/db/cwc/eng/cwc_annex_verification_part_VIII.html Annex on Implementation and Verification ("Verification Annex")<!-- Bot generated title -->]</ref> 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 (nerve agent)|tabun]].
====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 clean oils off of metals may also react under the UV created in a welding arc 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 [[polyurethane]]s. Significant amounts are also used in the production of [[polycarbonate]]s via its reaction with [[bisphenol A]].<ref name=Ullmann/> [[Polycarbonate]]s 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.<ref> Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. DOI: 10.1002/047084289X.rp149.</ref> Following are the three most useful reactions involving phosgene.
====Synthesis of carbonates====
[[Diol]]s react with phosgene to give either linear or cyclic carbonates (R = H, alkyl, aryl):
:HOCR<sub>2</sub>-X-CR<sub>2</sub>OH + COCl<sub>2</sub> → 1/n [OCR<sub>2</sub>-X-CR<sub>2</sub>OC(O)-]<sub>n</sub> + 2 HCl
====Synthesis of isocyanates====
The synthesis of [[isocyanate]]s from amines illustrates the [[electrophilic]] character of this reagent and its use in introducing the equivalent of "CO<sup>2+</sup>" (R = [[alkyl]], [[aryl]]):
<ref>{{OrgSynth | author = R. L. Shriner, W. H. Horne, and R. F. B. Cox | title = p-Nitrophenyl Isocyanate| collvol = 2 | collvolpages = 453| year = 1943 | prep = CV2P0453}}</ref>
:RNH<sub>2</sub> + COCl<sub>2</sub> → 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 chloride]]s from [[carboxylic acid]]s:
:RCO<sub>2</sub>H + COCl<sub>2</sub> → RC(O)Cl + HCl + [[Carbon dioxide|CO<sub>2</sub>]]
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 + COCl<sub>2</sub> → ROC(O)Cl + HCl
===Inorganic chemistry===
Although it is somewhat hydrophobic, phosgene reacts with [[water]] to releases [[hydrogen chloride]] and [[carbon dioxide]]:
:COCl<sub>2</sub> + H<sub>2</sub>O → CO<sub>2</sub> + 2 HCl
Analogously, with ammonia, one obtains [[urea]]:
:COCl<sub>2</sub> + 4 NH<sub>3</sub> → CO(NH<sub>2</sub>)<sub>2</sub> + 2 NH<sub>4</sub>Cl
Halide exchange with [[nitrogen trifluoride]] and [[aluminium tribromide]] gives COF<sub>2</sub> and COBr<sub>2</sub>, respectively.<ref name=Ullmann/>
==History==
Phosgene was synthesized by the chemist [[John Davy (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 language|Greek]], ''phos'' (light) and ''gene'' (born).<ref>{{cite journal
| title = On a Gaseous Compound of Carbonic Oxide and Chlorine
| author = John Davy
| journal = Philosophical Transactions of the Royal Society of London
| volume = 102
| year = 1812
| pages = 144–151
| url = http://links.jstor.org/sici?sici=0261-0523%281812%29102%3C144%3AOAGCOC%3E2.0.CO%3B2-1
| doi = 10.1098/rstl.1812.0008 }}</ref> It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.
===Chemical warfare===
{{further|[[Use of poison gas in World War I]]}}
Phosgene was stockpiled by various countries as part of their military arsenals until well after [[World War II]].<ref name=FM 3-8>FM 3-8 Chemical Reference handbook; US Army; 1967</ref> The United States began disposing of its stockpiles in 1969. Even before then, the importance of phosgene as a weapon had declined as the more lethal [[nerve agent]]s entered stockpiles. On August 24th, 2007, vials of purported phosgene were found near the United Nations headquarters in New York City, where the sample had been forgotten after being retrieved from Iraq in 1996. The FBI helped remove the chemicals and there was no danger. Preliminary sampling indicates a non-threatening agent (industrial solvent) as first reported by The Daily News (NYTimes.com, September 5, 2007).
===Bhopal disaster===
{{further|[[Bhopal disaster]]}}
In 1984 in Bhopal, India 43 tonnes of [[methylisocyanate]] was accidentally released, killing or injuring tens of thousands of people. One of the components of the leak was probably phosgene.
==Safety==
Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.<ref>{{cite journal
| title = Phosgene exposure: mechanisms of injury and treatment strategies
| author = Borak J., Diller W. F.
| journal = Journal of Occupational and Environmental Medicine
| year = 2001
| volume = 43
| issue = 2
| pages = 110–9
| pmid = 11227628
| doi = 10.1097/00043764-200102000-00008 <!--Retrieved from CrossRef by DOI bot-->
}}</ref>
Phosgene can be detected at 0.4 ppm, which is 4x 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 and causing suffocation. Phosgene detection badges are worn by those at risk of exposure.<ref name=Ullmann/>
[[Sodium bicarbonate]] may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with [[ammonia]].<ref>{{cite web | publisher = [[International Programme on Chemical Safety]] | title = Phosgene: Health and Safety Guide | year = 1998 | url = http://www.inchem.org/documents/hsg/hsg/hsg106.htm}}</ref>
==References==
<references/>
==External links==
*[http://web.lemoyne.edu/~giunta/jdavy.html Davy's account of his discovery of phosgene]
*[http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc00/icsc0007.htm International Chemical Safety Card 0007]
*[http://www.cdc.gov/niosh/npg/npgd0504.html NIOSH Pocket Guide to Chemical Hazards]
*[http://www.bt.cdc.gov/agent/phosgene/ U.S. CDC Emergency Preparedness & Response]
*[http://www.epa.gov/oppt/aegl/results7.htm U.S. EPA Acute Exposure Guideline Levels]
*[http://www.opcw.org/html/db/cwc/eng/cwc_annex_verification_part_VIII.html Regime For Schedule 3 Chemicals And Facilities Related To Such Chemicals], OPCW website
*[http://cbwinfo.com/Chemical/Pulmonary/CG.shtml CBWInfo website]
*[http://www.jlaw.com/Articles/NaziMedEx.html Use of Phosgene in WWII and in modern-day warfare] (Refer to Section 4.C of the article)
*http://www.vngas.com/pdf/g67.pdf
{{chemical warfare}}
<!--Categories-->
[[Category:Chemical weapons]]
[[Category:Inorganic carbon compounds]]
[[Category:Chlorides]]
[[Category:Acyl halides]]
[[Category:Nonmetal halides]]
<!--Interwiki-->
[[be:Фасген]]
[[cs:Fosgen]]
[[de:Phosgen]]
[[et:Fosgeen]]
[[el:Φωσγένιο]]
[[es:Fosgeno]]
[[fr:Phosgène]]
[[it:Fosgene]]
[[lv:Fosgēns]]
[[hu:Foszgén]]
[[nl:Fosgeen]]
[[ja:ホスゲン]]
[[pl:Fosgen]]
[[pt:Fosgênio]]
[[ro:Fosgen]]
[[ru:Фосген]]
[[sk:Fosgén]]
[[sl:Fosgen]]
[[fi:Fosgeeni]]
[[sv:Fosgen]]
[[zh:光气]]