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Phosgene (Molecule of the Month for September 2007)

Carbonyl chloride, Dichloroformaldehyde, Chemical Weapon

Phosgene 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. 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). It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Phosgene was first used by France under the direction of French chemist Victor Grignard in 1915. 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. Although Phosgene was never as notorious as mustard gas, it killed far more people, about 85% of the 100,000 deaths caused by chemical weapons during World War I. The toxicity of phosgene is mainly due to the HCl that is released in its reaction with water, especially in the lungs. Death by gas was particularly horrific. According to Denis Winter (Death's Men, 1978), a fatal dose of phosgene eventually led to "shallow breathing and retching, pulse up to 120, an ashen face and the discharge of four pints (2 liters) of yellow liquid from the lungs each hour for the 48 of the drowning spasms."

Phosgene is now used chiefly in the production of polymers including polyurethanes, polycarbonates, and polyureas. It is also valuable in the preparation of fine chemicals. 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

Picture of Phosgene

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Phosgene structure
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Picture of Phosgene

C O Cl2

Update by Karl Harrison
(Molecule of the Month for September 2007 )