Phosgene

Phosgene is a highly reactive toxic chemical compound with the formula CCl₂O.

This gas gained infamy as a chemical weapon during World War I, but it is also a valuable industrial reagent and building block in organic synthesis. It is colourless, but can appear as a white or yellowish haze when released into air, due to refraction of light. In low concentrations, its odor resembles recently cut hay or green corn (maize); at higher concentrations, it may be strongly unpleasant. In addition to its industrial production, small amounts occur naturally from the breakdown of chlorinated compounds and the combustion of chlorine-containing organic compounds.

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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°.^[1]

Phosgene is the simplest and one of the most electrophilic acid chlorides. This high electrophilicity is manifested in the tendency of phosgene to react with water, that is, hydrolyze. This hydrolysis reaction releases hydrogen chloride and carbon dioxide:

COCl₂ + H₂O → CO₂ + 2 HCl

The toxicity of phosgene is mainly due to the HCl that is released in this hydrolysis reaction.

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).^[2] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Further information: Use of poison gas in World War I

Phosgene was stockpiled as part of U.S. military arsenals until well after World War II in the form of aerial bombs and mortar rounds.^[3] 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 agents 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).

Production

Around 2 million tons are produced annually^[4] for use in the synthesis of fine chemicals and polymers. Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of highly porous carbon, which acts as a catalyst. The chemical equation for this reaction follows:

CO + Cl₂ → COCl₂

The reaction is exothermic, therefore the reactor must be cooled to carry away the heat it produces. Typically, the reaction is conducted between 50 and 150 °C. Above 200 °C, phosgene decomposes back into carbon monoxide and chlorine.

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.

Because of safety issues, phosgene is almost always produced and consumed within the same plant. 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 much less dangerous than nerve agents, phosgene is still regarded as a viable chemical warfare agent.

Uses

Phosgene is used chiefly in the production of polymers including polyurethanes, polycarbonates, and polyureas. It is also valuable in the preparation of fine chemicals.^[6] In the laboratory for small-scale reactions, gaseous phosgene has increasingly been supplanted by more easily handled reagents that effect comparable transformations: diphosgene (chloroformic acid ester), which is a liquid at room temperature, or triphosgene, a crystalline substance. Following are three of many 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

Polycarbonates are an important class of engineering thermoplastic, found for example in lenses in eye glasses.

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):

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 and esters

It is also used to produce acid chlorides:

RCO₂H + COCl₂ → RC(O)Cl + HCl + CO₂

Such acid chlorides react with amines and alcohols to give respectively amides and esters, which are common intermediates in the dye, pesticide, and pharmaceutical industries. Despite being an efficient method of synthesizing acyl chloride from carboxylic acids, laboratory safety issues led to the use of the less toxic thionyl chloride.

Safety

See MSDS. Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.^[7] Like many reactive chlorides, phosgene combines with water in the tissues of the respiratory tract to form hydrochloric acid. Phosgene is stable when stored in dry steel containers.^[3]. Phosgene is a member of a class of organic chemicals known as acylating agents.^{[citation needed]} These agents can react with both DNA and with enzymes (polymerases) that are responsible for replication of DNA in cells.

References

1. ^ Nakata, M.; Kohata, K.; Fukuyama, T.; Kuchitsu, K. “Molecular Structure of Phosgene as Studied by Gas Electron Diffraction and Microwave Spectroscopy. The r_z Structure and Isotope Effect ” Journal of Molecular Spectroscopy 1980, Volume 83, Pages 105-117. doi:10.1016/0022-2852(80)90314-8
2. ^ John Davy (1812). "On a Gaseous Compound of Carbonic Oxide and Chlorine". Philosophical Transactions of the Royal Society of London 102: 144-151.
3. ^ ^{a b} FM 3-8 Chemical Reference handbook; US Army; 1967
4. ^ http://cbwinfo.com/Chemical/Pulmonary/CG.shtml
5. ^ http://www.opcw.org/html/db/cwc/eng/cwc_annex_verification_part_VIII.html
6. ^ Hamley, P. "Phosgene" Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley, New York. DOI: 10.1002/047084289X.rp149.
7. ^ Borak J., Diller W. F. (2001). "Phosgene exposure: mechanisms of injury and treatment strategies". Journal of Occupational and Environmental Medicine 43 (2): 110-9. PMID 11227628.