Everything about Phosgene totally explained
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Phosgene is the
chemical compound with the
formula Cl
2CO. This colourless gas gained infamy as a
chemical weapon during
World War I, but it's 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 compounds. Approximately 5000 Mg were produced in 1989.
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°. It is one of the simplest
acid chlorides, 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: Although much less dangerous than most other chemical weapons (for example
mustard gas), phosgene is still regarded as a viable chemical warfare agent because it's so easy to manufacture when compared to the production requirements of more technically advanced chemical weapons such as the first-generation
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
polyurethanes. Significant amounts are also used in the production of
polycarbonates via its reaction with
bisphenol A. Following are the three most useful reactions involving phosgene.
Synthesis of carbonates
Diols react with phosgene to give either linear or cyclic carbonates (R = H, alkyl, aryl):
» HOCR
2-X-CR
2OH + COCl
2 → 1/n [OCR
2-X-CR
2OC(O)-]
n + 2 HCl
Synthesis of isocyanates
The synthesis of
isocyanates from amines illustrates the
electrophilic character of this reagent and its use in introducing the equivalent of "CO
2+" (R =
alkyl,
aryl):
» RNH
2 + COCl
2 → 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 chlorides from
carboxylic acids:
» RCO
2H + COCl
2 → RC(O)Cl + HCl +
CO2
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
2 → ROC(O)Cl + HCl
Inorganic chemistry
Although it's somewhat hydrophobic, phosgene reacts with
water to releases
hydrogen chloride and
carbon dioxide:
» COCl
2 + H
2O → CO
2 + 2 HCl
Analogously, with ammonia, one obtains
urea:
» COCl
2 + 4 NH
3 → CO(NH
2)
2 + 2 NH
4Cl
Halide exchange with
nitrogen trifluoride and
aluminium tribromide gives COF
2 and COBr
2, respectively. It gradually became important in the chemical industry as the
19th century progressed, particularly in dye manufacturing.
Chemical warfare
Phosgene was stockpiled by various countries as part of their military arsenals until well after
World War II. 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).
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.
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.
[Further Information]
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