Phosgene: working with, chemistry and safety
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. In addition to its industrial production, small amounts occur naturally from the breakdown and the combustion of organochlorine compounds, such as those used in refrigeration systems. The name, sounding similar to "phosphine", does not mean it has any phosphorus. The chemical was named by combining the Greek words ‘phos’ (meaning light) and genesis (birth).
Structure and basic propertiesPhosgene 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.
ProductionIndustrially, phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst:
- CO + Cl2 → COCl2 (ΔHrxn = −107.6kJ/mol)
The reaction is exothermic, therefore the reactor must be cooled. Typically, the reaction is conducted between 50 and 150 °C. Above 200 °C, phosgene reverts to carbon monoxide and chlorine, Keq (300K) = 0.05. Approximately 5000 tonnes were produced in 1989.
Because of safety issues, phosgene is almost always produced and consumed within the same plant and extraordinary measures are made to contain this toxic gas. 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. Although less dangerous than many other chemical weapons, such as sarin, 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 occurrenceUpon ultraviolet (UV) 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 remove oil from metals, such as automotive brake cleaners, are converted to phosgene by the UV rays of arc welding processes.
Phosgene may also be produced during testing for leaks of older-style refrigerant gasses. Chloromethanes (R12, R22 and others) were formerly leak-tested in situ by employing a small gas torch (propane, butane or propylene gas) with a sniffer tube and a copper reaction plate in the flame nozzle of the torch. If any refrigerant gas was leaking from a pipe or joint, the gas would be sucked into the flame via the sniffer tube and would cause a colour change of the gas flame to a bright greenish blue. In the process, phosgene gas would be created due to the thermal reaction. No valid statistics are available, but anecdotal reports suggest that numerous refrigeration technicians suffered the effects of phosgene poisoning due to their ignorance of the toxicity of phosgene, produced during such leak testing. Electronic sensing of refrigerant gases phased out the use of flame testing for leaks in the 1980s. Similarly, phosgene poisoning is a consideration for people fighting fires that are occurring in the vicinity of freon refrigeration equipment, smoking in the vicinity of a freon leak, or fighting fires using halon or halotron.
UsesThe 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.
Synthesis of carbonatesSignificant amounts are also used in the production of polycarbonates via its reaction with bisphenol A. Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eye glasses. Diols react with phosgene to give either linear or cyclic carbonates (R = H, alkyl, aryl):
- HOCR2-X-CR2OH + COCl2 → 1/n n + 2 HCl
Synthesis of isocyanatesThe synthesis of isocyanates from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent of "CO2+":
- RNH2 + COCl2 → RN=C=O + 2 HCl (R = alkyl, aryl)
Laboratory usesIn 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. Aside from the above reactions that are widely practiced industrially, phosgene is also used to produce acid chlorides and carbon dioxide from carboxylic acids:
- RCO2H + COCl2 → RC(O)Cl + HCl + CO2
- ROH + COCl2 → ROC(O)Cl + HCl
Other chemistryAlthough it is somewhat hydrophobic, phosgene reacts with water to release hydrogen chloride and carbon dioxide:
- COCl2 + H2O → CO2 + 2 HCl
- COCl2 + 4 NH3 → CO(NH2)2 + 2 NH4Cl
HistoryPhosgene was synthesized by the British chemist John Davy (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it "phosgene" in reference of the 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.
Chemical warfareFollowing the extensive use of phosgene gas in combat during World War I, it was stockpiled by various countries as part of their secret chemical weapons programs.
SafetyPhosgene 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. It reacts with the amines of the proteins, causing crosslinking via formation of urea-like linkages, in accord with the reactions discussed above. Phosgene detection badges are worn by those at risk of exposure.
Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia.