AskDefine | Define phosgene

Dictionary Definition

phosgene n : a colorless poisonous gas that smells like new-mown hay; used in chemical warfare

User Contributed Dictionary

English

Etymology

From phos- + -gen.

Pronunciation

  • /ˈfɒsdʒiːn/, /ˈfɒzdʒiːn/

Noun

  1. alternative form of carbonyl chloride
    • 2006, Thomas Pynchon, Against the Day, Vintage 2007, p. 265:
      “‘Poison gas’?” A new one on Lew. But Dr. De Bottle had taken on a somber look. “Phosgene.”

Extensive Definition

Phosgene is the chemical compound with the formula Cl2CO. This colourless gas gained infamy as a chemical weapon during World War I, but it is 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 (e.g. mustard gas), phosgene is still regarded as a viable chemical warfare agent because it is 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):
HOCR2-X-CR2OH + COCl2 → 1/n [OCR2-X-CR2OC(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 "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

It is also used to produce acid chlorides from carboxylic acids:
RCO2H + COCl2 → 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 + COCl2 → ROC(O)Cl + HCl

Inorganic chemistry

Although it is somewhat hydrophobic, phosgene reacts with water to releases hydrogen chloride and carbon dioxide:
COCl2 + H2O → CO2 + 2 HCl
Analogously, with ammonia, one obtains urea:
COCl2 + 4 NH3 → CO(NH2)2 + 2 NH4Cl
Halide exchange with nitrogen trifluoride and aluminium tribromide gives COF2 and COBr2, respectively. It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Chemical warfare

further Use of poison gas in World War I 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

further 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.

References

phosgene in Belarusian: Фасген
phosgene in Czech: Fosgen
phosgene in German: Phosgen
phosgene in Estonian: Fosgeen
phosgene in Modern Greek (1453-): Φωσγένιο
phosgene in Spanish: Fosgeno
phosgene in French: Phosgène
phosgene in Italian: Fosgene
phosgene in Latvian: Fosgēns
phosgene in Dutch: Fosgeen
phosgene in Japanese: ホスゲン
phosgene in Polish: Fosgen
phosgene in Portuguese: Fosgênio
phosgene in Romanian: Fosgen
phosgene in Russian: Фосген
phosgene in Slovak: Fosgén
phosgene in Slovenian: Fosgen
phosgene in Finnish: Fosgeeni
phosgene in Swedish: Fosgen
phosgene in Chinese: 光气
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