Phosgene

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°.[9] Phosgene is a carbon oxohalide and it can be considered one of the simplest acyl chlorides, being formally derived from carbonic acid.

Industrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:[8]

CO + Cl₂ → COCl₂ (ΔH_rxn = −107.6 kJ/mol)

This reaction is exothermic and is typically performed between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, K_eq(300 K) = 0.05. World production of this compound was estimated to be 2.74 million tonnes in 1989.[8]

Phosgene is fairly simple to produce, but is listed as a Schedule 3 substances under the Chemical Weapons Convention. As such, it is usually considered too dangerous to transport in bulk quantities. Instead, phosgene is usually produced and consumed within the same plant, as part of an "on demand" process. This involves maintaining equivalent rates of production and consumption, which keeps the amount of phosgene in the system at any one time fairly low, reducing the risks in the event of an accident. Some batch production does still take place, but efforts are made to reduce the amount of phosgene stored.[10]

Phosgene was synthesized by the Cornish chemist John Davy (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it "phosgene" from Greek φῶς (phos, light) and γεννάω (gennaō, to give birth) in reference of the use of light to promote the reaction.[15] It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

The reaction of an organic substrate with phosgene is called phosgenation.[8]

Chemical warfare

US Army phosgene identification poster from World War II

Further information: Chemical weapons in World War I and Second Italo-Ethiopian War

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.[19] Although less toxic than many other chemical weapons such as sarin, phosgene is still regarded as a viable chemical warfare agent because of its simpler manufacturing requirements when compared to that of more technically advanced chemical weapons such as tabun, a first-generation nerve agent.[20]

Phosgene was first deployed as a chemical weapon by the French in 1915 in World War I.[21] It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene.[22][23] Phosgene was more potent than chlorine, though some symptoms took 24 hours or more to manifest.

Following the extensive use of phosgene during World War I, it was stockpiled by various countries.[24][25][26]

Phosgene was then only infrequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.[27] Gas weapons, such as phosgene, were produced by Unit 731.

American Red Cross and Medical Research Committee, "An Atlas of Gas Poisoning" plate II, phosgene patient

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.[28]

The odor detection threshold for phosgene is 0.4 ppm, four times the threshold limit value. Its high toxicity arises from the action of the phosgene on the –OH, –NH₂ and –SH groups of the proteins in pulmonary alveoli (the site of gas exchange), respectively forming ester, amide and thioester functional groups in accord with the reactions discussed above. This results in disruption of the blood–air barrier, eventually causing pulmonary edema. The extent of damage in the alveoli does not primarily depend on phosgene concentration in the inhaled air, with the dose (amount of inhaled phosgene) being the critical factor.[29] Dose can be approximately calculated as "concentration" × "duration of exposure".[29][30] Therefore, persons in workplaces where there exists risk of accidental phosgene release usually wear indicator badges close to the nose and mouth.[31] Such badges indicate the approximate inhaled dose, which allows for immediate treatment if the monitored dose rises above safe limits. [31]

In case of low or moderate quantities of inhaled phosgene, the exposed person is to be monitored and subjected to precautionary therapy, then released after several hours. For higher doses of inhaled phosgene (above 150 ppm × min) a pulmonary edema often develops which can be detected by X-ray imaging and regressive blood oxygen concentration. Inhalation of such high doses can eventually result in fatality within hours up to 2–3 days of the exposure.

The risk connected to a phosgene inhalation is based not so much on its toxicity (which is much lower in comparison to modern chemical weapons like sarin or tabun) but rather on its typical effects: the affected person may not develop any symptoms for hours until an edema appears, at which point it could be too late for medical treatment to assist.[32] Nearly all fatalities as a result of accidental releases from the industrial handling of phosgene occurred in this fashion. On the other hand, pulmonary edemas treated in a timely manner usually heal in the mid- and longterm, without major consequences once some days or weeks after exposure have passed.[33][34] Nonetheless, the detrimental health effects on pulmonary function from untreated, chronic low-level exposure to phosgene should not be ignored; although not exposed to concentrations high enough to immediately cause an edema, many synthetic chemists (e.g. Leonidas Zervas) working with the compound were reported to experience chronic respiratory health issues and eventual respiratory failure from continuous low-level exposure.

If accidental release of phosgene occurs in an industrial or laboratory setting, it can be mitigated with ammonia gas; in the case of liquid spills (e.g. of diphosgene or phosgene solutions) an absorbent and sodium carbonate can be applied.[35]

• The first major phosgene-related incident happened in May 1928 when eleven tons of phosgene escaped from a war surplus store in central Hamburg.[36] Three hundred people were poisoned, of whom ten died.[36]
• In the second half of 20th century several fatal incidents implicating phosgene occurred in Europe, Asia and the US. Most of them have been investigated by authorities and the outcome made accessible to the public. For example, phosgene was initially blamed for the Bhopal disaster, but investigations proved methyl isocyanate to be responsible for the numerous poisonings and fatalities.
• Recent major incidents happened in January 2010 and May 2016. An accidental release of phosgene gas at a DuPont facility in West Virginia killed one employee in 2010.[37] The US Chemical Safety Board released a video detailing the accident.[38] Six years later, a phosgene leak occurred in a BASF plant in South Korea, where a contractor inhaled a lethal dose of phosgene.[39]

• Davy's account of his discovery of phosgene
• International Chemical Safety Card 0007
• CDC - Phosgene - NIOSH Workplace Safety and Health Topic
• NIOSH Pocket Guide to Chemical Hazards
• U.S. CDC Emergency Preparedness & Response
• U.S. EPA Acute Exposure Guideline Levels
• Regime For Schedule 3 Chemicals And Facilities Related To Such Chemicals, OPCW website
• CBWInfo website
• Use of Phosgene in WWII and in modern-day warfare