Phosgene

Phosgene

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Phosgene is the chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 COCl2. This colorless gas gained infamy as a chemical weapon during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. It is also a valued industrial reagent and building block in synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

 of pharmaceuticals and other organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s. 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
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 of organochlorine compound
Organochloride
An organochloride, organochlorine, chlorocarbon, chlorinated hydrocarbon, or chlorinated solvent is an organic compound containing at least one covalently bonded chlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications...

s, such as those used in refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...

 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 properties


Phosgene is a planar molecule as predicted by VSEPR theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...

. 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
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

.

Production


Industrially, phosgene is produced by passing purified carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 and chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 gas through a bed of porous activated carbon
Activated carbon
Activated carbon, also called activated charcoal, activated coal or carbo activatus, is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions.The word activated in the name is sometimes replaced...

, 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
Chemical Weapons Convention
The Chemical Weapons Convention is an arms control agreement which outlaws the production, stockpiling and use of chemical weapons. Its full name is the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction...

: 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
Sarin
Sarin, or GB, is an organophosphorus compound with the formula [2CHO]CH3PF. It is a colorless, odorless liquid, which is used as a chemical weapon. It has been classified as a weapon of mass destruction in UN Resolution 687...

, 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
Nerve agent
Nerve agents are a class of phosphorus-containing organic chemicals that disrupt the mechanism by which nerves transfer messages to organs...

 tabun
Tabun (nerve agent)
Tabun or GA is an extremely toxic chemical substance. It is a clear, colorless, and tasteless liquid with a faint fruity odor. It is classified as a nerve agent because it fatally interferes with normal functioning of the mammalian nervous system...

.

Adventitious occurrence


Upon ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (UV) radiation in the presence of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...

 slowly converts into phosgene via a radical reaction. To suppress this photodegradation
Photodegradation
Photodegradation is degradation of a photodegradable molecule caused by the absorption of photons, particularly those wavelengths found in sunlight, such as infrared radiation, visible light, and ultraviolet light. However, other forms of electromagnetic radiation can cause 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
Arc welding
Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes...

 processes.

Phosgene may also be produced during testing for leaks of older-style refrigerant gasses. Chloromethanes
Chloromethane
Chloromethane, also called methyl chloride, R-40 or HCC 40, is a chemical compound of the group of organic compounds called haloalkanes. It was once widely used as a refrigerant. It is a colorless extremely flammable gas with a minorly sweet odor, which is, however, detected at possibly toxic levels...

 (R12
Dichlorodifluoromethane
Dichlorodifluoromethane , is a colorless gas, and usually sold under the brand name Freon-12, is a chlorofluorocarbon halomethane , used as a refrigerant and aerosol spray propellant. Complying with the Montreal Protocol, its manufacture was banned in the United States along with many other...

, R22
Chlorodifluoromethane
Chlorodifluoromethane or difluoromonochloromethane is a hydrochlorofluorocarbon . This colorless gas is better known as HCFC-22, or R-22. It was once commonly used as a propellant and in air conditioning applications...

 and others) were formerly leak-tested in situ by employing a small gas torch (propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

, butane
Butane
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...

 or propylene
Propylene
Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...

 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
Halomethane
Halomethane compounds are derivatives of methane with one or more of the hydrogen atoms replaced with halogen atoms . Halomethanes are both naturally occurring, especially in marine environments, and man-made, most notably as refrigerants, solvents, propellants, and fumigants...

 or halotron.

Uses


The great majority of phosgene is used in the production of isocyanate
Isocyanate
Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...

s, the most important being toluene diisocyanate
Toluene diisocyanate
Toluene diisocyanate is an aromatic diisocyanate. It is produced for reaction with polyols to form polyurethanes. It exists in two isomers, 2,4-TDI and 2,6-TDI . 2,4-TDI is produced in the pure state, but TDI is often marketed as 80/20 and 65/35 mixtures of the 2,4 and 2,6 isomers respectively...

 (TDI) and methylene diphenyl diisocyanate
Methylene diphenyl diisocyanate
Methylene diphenyl diisocyanate, most often abbreviated as MDI, is an aromatic diisocyanate. It exists in three isomers, 2,2'-MDI, 2,4'-MDI, and 4,4'-MDI, but the 4,4' isomer is most widely used. This isomer is also known as Pure MDI. MDI reacts with polyols in the manufacture of polyurethane...

 (MDI). These isocyanates are precursors to polyurethane
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...

s. Significant amounts are also used in the production of polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

s via its reaction with bisphenol A
Bisphenol A
Bisphenol A is an organic compound with two phenol functional groups. It is used to make polycarbonate plastic and epoxy resins, along with other applications....

. Polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

s are an important class of engineering thermoplastic
Thermoplastic
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a liquid when heated and freezes to a very glassy state when cooled sufficiently...

 found, for example, in lenses in eye glasses.

Organic synthesis


Although phosgene still finds use in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

, a variety of substitutes have been developed, notably trichloromethyl chloroformate (“diphosgene
Diphosgene
Diphosgene is a chemical compound with the formula ClCO2CCl3. This colorless liquid is a valuable reagent in the synthesis of organic compounds...

”), which is a liquid at room temperature, and bis(trichloromethyl) carbonate (“triphosgene
Triphosgene
Triphosgene is a chemical compound that is used as a safer substitute for phosgene, because at room temperature it is a solid crystal, as opposed to phosgene which is a gas. Triphosgene crystals decompose at around 130 °C, although, the decomposition temperature of impure samples can be much lower...

”), a crystalline substance. The following are the three most useful reactions involving phosgene.

Synthesis of carbonates


Diol
Diol
A diol or glycol is a chemical compound containing two hydroxyl groups A geminal diol has two hydroxyl groups bonded to the same atom...

s 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 isocyanate
Isocyanate
Isocyanate is the functional group of elements –N=C=O , not to be confused with the cyanate functional group which is arranged as –O–C≡N or with isocyanide, R-N≡C. Any organic compound which contains an isocyanate group may also be referred to in brief as an isocyanate. An isocyanate may have more...

s from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent of "CO2+" (R = alkyl, aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....

):
RNH2 + COCl2 → RN=C=O + 2 HCl

Such reactions are conducted in the presence of a base such as pyridine
Pyridine
Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

 that absorbs the hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

.

Synthesis of acid chlorides


It is also used to produce acid chlorides and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 from carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s:
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
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...

 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
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to release hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

:
COCl2 + H2O → CO2 + 2 HCl

Analogously, with ammonia, one obtains urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

:
COCl2 + 4 NH3 → CO(NH2)2 + 2 NH4Cl

Halide exchange with nitrogen trifluoride
Nitrogen trifluoride
Nitrogen trifluoride is the inorganic compound with the formula NF3. This nitrogen-fluorine compound is a colorless, toxic, odourless, nonflammable gas. It finds increasing use as an etchant in microelectronics.-Applications:...

 and aluminium tribromide gives COF2 and COBr2, respectively.

History


Phosgene was synthesized by the British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 chemist John Davy
John Davy (chemist)
John Davy FRS was a British doctor, amateur chemist, and brother of the noted chemist Sir Humphry Davy, and cousin of Edmund Davy....

 (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. He named it "phosgene" in reference of the use of light to promote the reaction; from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

, phos (light) and gene (born). It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing.

Chemical warfare



Following the extensive use of phosgene gas in combat during World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, it was stockpiled by various countries as part of their secret chemical weapons programs.

Phosgene was then only frequently used by the Imperial Japanese Army
Imperial Japanese Army
-Foundation:During the Meiji Restoration, the military forces loyal to the Emperor were samurai drawn primarily from the loyalist feudal domains of Satsuma and Chōshū...

 against the Chinese during the Second Sino-Japanese War
Second Sino-Japanese War
The Second Sino-Japanese War was a military conflict fought primarily between the Republic of China and the Empire of Japan. From 1937 to 1941, China fought Japan with some economic help from Germany , the Soviet Union and the United States...

. Gas weapons, such as phosgene, were produced by Unit 731
Unit 731
was a covert biological and chemical warfare research and development unit of the Imperial Japanese Army that undertook lethal human experimentation during the Second Sino-Japanese War and World War II. It was responsible for some of the most notorious war crimes carried out by Japanese...

 and authorized by specific orders given by Hirohito (Emperor Showa) himself, transmitted by the chief of staff of the army
Imperial General Headquarters
The as part of the Supreme War Council was established in 1893 to coordinate efforts between the Imperial Japanese Army and Imperial Japanese Navy during wartime...

. For example, the Emperor authorized the use of toxic gas on 375 separate occasions during the battle of Wuhan
Battle of Wuhan
The Battle of Wuhan, popularly known to the Chinese as the Defence of Wuhan, and to the Japanese as the Capture of Wuhan, was a large-scale battle of the Second Sino-Japanese War...

 from August to October 1938.

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 four times the Threshold Limit Value
Threshold Limit Value
The threshold limit value of a chemical substance is a level to which it is believed a worker can be exposed day after day for a working lifetime without adverse health effects. Strictly speaking, TLV is a reserved term of the American Conference of Governmental Industrial Hygienists...

. Its high toxicity
Toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 arises from the action of the phosgene on the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s in the pulmonary alveoli, which are the site of gas exchange: their damage disrupts the blood-air barrier, causing suffocation. It reacts with the amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

s of the proteins, causing crosslinking via formation of urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

-like linkages, in accord with the reactions discussed above. Phosgene detection badges are worn by those at risk of exposure.

Sodium bicarbonate
Sodium bicarbonate
Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...

 may be used to neutralise liquid spills of phosgene. Gaseous spills may be mitigated with ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

.

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