Phosgene

Upon ultraviolet (UV) radiation in the presence of oxygen, chloroform slowly converts into phosgene by a radical reaction. To suppress this photodegradation, chloroform is often stored in brown-tinted glass containers and with a small percentage of ethanol added. Chlorinated solvents used to remove oil from metals, such as automotive brake cleaners, are converted to phosgene by the UV rays of arc welding processes.[11]

Phosgene may also be produced during testing for leaks of older-style refrigerant gases. 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 through 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 possibility for people fighting fires that occur in the vicinity of refrigerant leaks from air-conditioning systems or refrigeration equipment, smoking in the vicinity of a freon refrigerant leak, or fighting fires using halon or halotron.

Phosgene can be released during building fires. In one instance, a deputy fire chief died ten days after inhaling fumes that wafted down outside a burning restaurant. After a two-day hospitalization he had appeared to recover, but ultimately suffered cardiac arrest at home following tracheobronchial inflammation, alveolar hemorrhage, and pulmonary edema. The phosgene was produced by decomposing Freon 22 after flames ducted up from a grease fire heated an air-conditioning unit on the roof and ruptured a hose.[12]

Significant amounts are also used in the production of polycarbonates by its reaction with bisphenol A.[7] Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eyeglasses. 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

The synthesis of isocyanates from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent of "CO²⁺":[14]

RNH₂ + COCl₂ → RN=C=O + 2 HCl (R = alkyl, aryl)

Such reactions are conducted in the presence of a base such as pyridine that absorbs the hydrogen chloride.

In the research laboratory phosgene still finds limited use in organic synthesis. A variety of substitutes have been developed, notably trichloromethyl chloroformate ("diphosgene"), a liquid at room temperature, and bis(trichloromethyl) carbonate ("triphosgene"), a crystalline substance.[15] Aside from the above reactions that are widely practiced industrially, phosgene is also used to produce acyl chlorides and carbon dioxide 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

Phosgene is stored in metal cylinders. The outlet is always standard, a tapered thread that is known as CGA 160

Although it is somewhat hydrophobic, phosgene reacts with water to release 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.[7]

US Army phosgene identification poster from World War II

The collapse of international conventions against chemical weapons led to the widespread use of chlorine gas in World War I, but its lethal concentration of 0.1% was visible as a green cloud in the air, allowing troops to take readily available countermeasures. Phosgene, colorless with a more subtle "moldy hay" odor, was introduced by a group of French chemists led by Victor Grignard and first used by the French in 1915.[16] It was also used in a mixture with an equal volume of chlorine, with the chlorine helping to spread the denser phosgene.[17][18] Phosgene was more potent than chlorine, though some of the symptoms of exposure took 24 hours or more to manifest, meaning the victims were initially still capable of putting up a fight.[19]

Following 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.[20][21][22]

In May 1928, eleven tons of phosgene escaped from a war surplus store in central Hamburg.[23] Three hundred people were poisoned, of whom 10 died.[23]

Phosgene was then only infrequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.[24] Gas weapons, such as phosgene, were produced by Unit 731 and authorized by specific orders given by Hirohito (Emperor Showa) himself, transmitted by the chief of staff of the army. For example, the Emperor authorized the use of toxic gas on 375 separate occasions during the Battle of Wuhan from August to October 1938.[25]

• On January 23, 2010, an accidental release of phosgene gas at a DuPont facility in West Virginia killed one employee. [28]