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Phosgene
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Phosgene is the chemical compound with the formula COCl2. This colorless
gas gained infamy as a chemical weapon during World War I. It is also a
valued industrial reagent and building block in synthesis of
pharmaceuticals and other organic compounds. In low concentrations, its
odor resembles freshly cut hay or grass.
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Phosgene was first produced by John Davy (inventor of the ‘Davy Lamp’)
at the start of the 19 th century. The chemical was named by combining
the Greek words ‘phos’ (meaning light) and genesis (birth), as Davy used
sunlight to produce the chemical from carbon monoxide and chlorine.
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Phosgene is produced by passing purified carbon monoxide and chlorine
gas through a bed of porous activated carbon, which serves as a
catalyst.
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Phosgene may also be produced during testing for leaks of older-style
refrigerant gasses.
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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.
Phosgene was used during World War I as a choking (pulmonary) agent.
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In the
research laboratory phosgene still finds limited 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.
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Phosgene is an insidious poison as the odor may not be noticed and
symptoms may be slow to appear. Phosgene can be detected at 0.4 ppm,
which is four times the Threshold Limit Value. Its high toxicity arises
from the action of the phosgene on the proteins in the pulmonary
alveoli, which are the site of gas exchange: their damage disrupts the
blood-air barrier, causing suffocation.
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The
primary use of phosgene is in the production of toluene diisocyanate (TDI),
a precursor of the polyurethane resins used to make foams, elastomers,
and coatings. A rapidly growing use of phosgene is in the manufacture of
polymethylene polyphenylisocyanate (PMPPI), which is used in the
production of rigid polyurethane foams.
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The
potential for phosgene generation by chlorocarbon decomposition exists
at chlorocarbon producing facilities, metallurgical operations,
drycleaning and degreasing facilities, certain types of industrial
fires, and wherever solvents contact heat or ultraviolet light.
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In all
commercial phosgene processes, the chlorine atoms react with active
hydrogen atoms to produce hydrogen chloride (HCl). Hydrogen chloride is
an acid gas, and like phosgene, it can be controlled with a caustic
scrubber; however, it is usually desirable for a water scrubber to
precede the caustic scrubber.
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Phosgene is unlikely to cause harm to the unborn child and is not
considered to be a cancer-causing chemical (carcinogen). Exposure to
phosgene has not been linked to the development of cancer. In other
words, phosgene is not thought to be carcinogenic.
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