Phosgene

Phosgene
General
Systematic name	Carbonyl chloride
Other names	Carbonic acid dichloride
Molecular formula	COCl₂
SMILES	O=C(Cl)Cl
Molar mass	98.9 g mol⁻¹
Appearance	colorless gas
CAS number	^*
Properties
Density and phase	4.248 g dm⁻³, gas (15 °C)
Solubility in water	Reacts
Melting point	−118 °C (155 K)
Boiling point	8 °C (281 K)
Structure
Molecular shape	Planar
Dipole moment	? D
Hazards
MSDS	http://www.vngas.com/pdf/g67.pdf
EU classification	Very toxic (T+)
NFPA 704
R-phrases	,
S-phrases	, , , ,
Flash point	non-flammable
RTECS number	SY5600000
Related compounds
Other anions	Carbonyl fluoride
Other cations	Nitrosyl chloride
Related compounds	Carbonic acid Urea Carbon monoxide Chloroformic acid
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Phosgene (also carbonyl chloride, COCl₂, or its military designation CG) is a non-flammable highly toxic gas or refrigerated liquid. It was used as a chemical weapon during World War I but is now a major industrial chemical with around 2 million tons produced annuallyhttp://cbwinfo.com/Chemical/Pulmonary/CG.shtml for use in the manufacture of isocyanates, polyurethane and polycarbonates, pesticides, herbicides and dyes.

Phosgene gas has no color but may appear as a white or yellowish haze when released into air. In lower concentrations its odor resembles recently cut hay or green corn (maize) while at higher concentrations it may be strong and unpleasant. The odor may not be noticed by all people exposed.

It is a manufactured chemical, but small amounts occur naturally from the breakdown of chlorinated compounds. Phosgene can also result from the combustion of chlorine-containing organic compounds.

Phosgene is a particularly insidious poison, as phosgene exposure often has no initial symptoms. Symptoms usually appear within 24 hours, but can take up to 72 hours to manifest. The gas combines with water in the tissues of the respiratory tract to form carbon dioxide and hydrochloric acid. The acid then dissolves the membranes in the lungs. Fluid fills the lungs, and death results from a combination of blood loss, shock, and respiratory failure. Phosgene is also an eye and skin irritant. It can also be a contaminant in food or water.

Phosgene is a member of a class of organic chemicals known as alkylating agents. These agents can react with both DNA and with enzymes (polymerases) that are responsible for replication of DNA in cells. As such, it is considered to be a carcinogen, even at low exposure levels.

Production and use

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 their reaction is

CO + Cl₂ → COCl₂

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

Because of the safety issues with storing and transporting it, phosgene is almost always produced and consumed within the same chemical 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.http://www.opcw.org/html/db/cwc/eng/cwc_annex_verification_part_VIII.html Although much less dangerous than nerve agents, phosgene is still regarded as a viable chemical warfare agent.

Phosgene is used chiefly in the production of polymers including polyurethanes, polycarbonates, and polyureas. It is also used to produce isocyanates and acid chlorides as intermediates in the dye, pesticide, and pharmaceutical industries. It is possible to use phosgene to isolate certain metals including aluminium and uranium from their ores, but these methods are not widely used.

Where phosgene is manufactured in situ for industrial processes, it is standard policy to only manufacture sufficient to maintain production processes in which the phosgene is consumed. This policy ensures that there is no need to store large amounts of phosgene on site, thereby removing the danger of a major leak into the surrounding area.

In the laboratory, the use of the gaseous phosgene has long since been replaced by diphosgene (Chloroformic acid ester), which is a liquid at room temperature, or triphosgene(Carbonic acid diester), a crystalline substance.

History

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

Use as a chemical weapon

It was first used as a weapon by the French, under the direction of French chemist Victor Grignard in 1915. Later, the Germans, under the direction of German chemist Fritz Haber added small quantities to chlorine to increase the latter's toxicity. Although phosgene was sometimes used on its own, it was more often used mixed with an equal volume of chlorine, the chlorine helping to spread the denser phosgene. The Allies called this combination White Star after the marking painted on shells containing the mixture).

In the first combined chlorine/phosgene attack by Germany, against British troops at Nieltje near Ypres, Belgium on 19 December 1915, 88 tons of the gas were released from cylinders causing 1069 casualties and 69 deaths.http://www.chemistry.usna.edu/plebechem/vsepr/chok.html The British P gas helmet, issued at the time, was impregnated with phenate hexamine and partially effective against phosgene. The modified PH Gas Helmet, which was additionally impregnated with hexamethylenetetramine to improve the protection against phosgene, was issued in January 1916.

Around 36,600 tons of the gas were manufactured during the war, out of a total of 190,000 tons for all chemical weapons, making it second only to chlorine (93,800 tons) in the quantity manufactured:http://www.mitretek.org/AShortHistoryOfChemicalWarfareDuringWorldWarI.htm

Germany 18,100 tons
France 15,700 tons
Great Britain 1,400 tons (although they also used French stocks)
United States 1,400 tons (although they also used French stocks)

Although it was never as notorious in public consciousness as mustard gas, it killed far more people, being responsible for about 85% of the 100,000 deaths caused by chemical weapons during World War I.

Phosgene was stockpiled as part of military arsenals until well after the Second world war, the United States only 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.

Safety

With ultraviolet radiation and oxygen present, chloroform can form significant amounts of phosgene in a radical reaction. Brown glass flasks for chloroform prevent this reaction.

References

External links

Chemical weapons | Inorganic carbon compounds | Chlorides | Acyl halides | Nonmetal halides