View source for Chemistry:Carbon monoxide

{{Short description|Colourless, odourless, tasteless and toxic gas}}
{{Chembox
|verifiedrevid  = 477004453
|ImageFileL1 = Carbon-monoxide-3D-balls.png
|ImageNameL1 = Ball-and-stick model of carbon monoxide
|ImageFileR1 = Carbon-monoxide-3D-vdW.png
|ImageNameR1 = Spamodel of carbon monoxide
|ImageFile2 = Carbon monoxide 2D.svg
|ImageSize2 = 170px
|ImageName2 = model of carbon monoxide
|IUPACName= Carbon monoxide
|OtherNames = Carbonic oxide gas<br />Carbon protoxide<br />Oxide of carbon<br />Protoxide of carbon<br />Carbonous oxide<br />Carbonous acid gas<br />Carbon(II) oxide<br />Breath of carbon<br />Oxygenated carbon<br />Carbate<br />Carbonyl<br/>Water gas<br/>Hydrocarbon gas<br/>Fuel gas<br/>Rauchgas<br />Carbonic inflammable air<br />Heavy inflammable air<br />White damp<br />Fire Damp<br />Powder Gas<br />Illuminating gas<br />Dowson gas<br />Mond gas<br />Power gas<br />Producer gas<br />Blast furnace gas<br />Coal gas<br />Phlogiston<br />Car gas
|Section1 = {{Chembox Identifiers
|InChI = 1/CO/c1-2
|InChIKey = UGFAIRIUMAVXCW-UHFFFAOYAT
|CASNo = 630-08-0
|CASNo_Ref = {{cascite|correct|CAS}}
|PubChem = 281
|ChemSpiderID = 275
|ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = 7U1EE4V452
|EINECS = 211-128-3
|UNNumber = 1016
|KEGG_Ref = {{keggcite|correct|kegg}}
|KEGG = D09706
|MeSHName = Carbon+monoxide
|ChEBI_Ref = {{ebicite|correct|EBI}}
|ChEBI = 17245
|ChEMBL = 1231840
|RTECS = FG3500000
|SMILES = [C-]#[O+]
|StdInChI_Ref = {{stdinchicite|correct|chemspider}}
|StdInChI = 1S/CO/c1-2
|StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
|StdInChIKey = UGFAIRIUMAVXCW-UHFFFAOYSA-N
|Beilstein = 3587264
|Gmelin = 421
}}
|Section2 = {{Chembox Properties
| C=1 | O=1
|Appearance = Colorless
|Odor = Odorless
|Density = {{ubl
 | 789&nbsp;kg/m<sup>3</sup>, liquid
 | 1.250&nbsp;kg/m<sup>3</sup> at 0&nbsp;°C, 1&nbsp;atm
 | 1.145&nbsp;kg/m<sup>3</sup> at 25&nbsp;°C, 1&nbsp;atm
}}
|Solubility = 27.6&nbsp;mg/L (25&nbsp;°C)
|SolubleOther = soluble in [[Chemistry:Chloroform|chloroform]], [[Chemistry:Acetic acid|acetic acid]], [[Chemistry:Ethyl acetate|ethyl acetate]], [[Chemistry:Ethanol|ethanol]], ammonium hydroxide, [[Chemistry:Benzene|benzene]]
|MeltingPtC = −205.02
|BoilingPtC = −191.5
|Dipole = 0.122 [[Debye|D]]
|RefractIndex = 1.0003364
|HenryConstant = 1.04 atm·m<sup>3</sup>/mol
|MagSus = −9.8·10<sup>−6</sup> cm<sup>3</sup>/mol
}}
|Section3 = {{Chembox Thermochemistry
|HeatCapacity = 29.1 J/(K·mol)
|Entropy = 197.7 J/(K·mol)
|DeltaHf = −110.5 kJ/mol
|DeltaHc = −283.0 kJ/mol
}}
|Section4 = {{Chembox Pharmacology
|ATCCode_prefix = V04
|ATCCode_suffix = CX08
}}
|Section5 = {{Chembox Hazards
|MainHazards = [[Chemistry:Poison|Poison]]ous by [[Biology:Inhalation|inhalation]]<ref name=PGCH/>
|ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0023.htm ICSC 0023]
|NFPA_ref = <ref>{{cite book|last1=Richard|first1=Pohanish|title=Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens|date=2012|publisher=Elsevier|isbn=978-1-4377-7869-4|page=572|edition=2|url=https://books.google.com/books?id=f6HclgoIkjcC&pg=PA572|access-date=5 September 2015}}</ref>
|NFPA-H = 3
|NFPA-F = 4
|NFPA-R = 0
|GHSPictograms = {{GHS02}}{{GHS06}}{{GHS08}}
|GHSSignalWord = Danger
|HPhrases = {{H-phrases|220|331|360|372|420}}
|PPhrases = {{P-phrases|201|202|210|260|261|264|270|25
1|281|304+340|308+313|311|314|321|377|381|403|403+233|405|501}}
|FlashPtC = −191
|AutoignitionPtC = 609
|ExploLimits = 12.5–74.2%
|NIOSH_ref = <ref name=PGCH>{{PGCH|0105}}</ref>
|PEL = TWA 50&nbsp;ppm (55&nbsp;mg/m<sup>3</sup>)
|REL = {{ubl
|TWA 35&nbsp;ppm (40&nbsp;mg/m<sup>3</sup>)
|C 200&nbsp;ppm (229&nbsp;mg/m<sup>3</sup>)
}}
|IDLH = 1200&nbsp;ppm
|LC50 = {{ubl
 | 8636&nbsp;ppm (rat, 15&nbsp;min)
 | 5207&nbsp;ppm (rat, 30&nbsp;min)
 | 1784&nbsp;ppm (rat, 4&nbsp;h)
 | 2414&nbsp;ppm (mouse, 4&nbsp;h)
 | 5647&nbsp;ppm (guinea pig, 4&nbsp;h)<ref name=IDLH>{{IDLH|630080|Carbon monoxide}}</ref>
}}
|LCLo = {{ubl
 | 4000&nbsp;ppm (human, 30&nbsp;min)
 | 5000&nbsp;ppm (human, 5&nbsp;min)<ref name=IDLH/>
}}
}}<ref>{{cite web|url=http://cameochemicals.noaa.gov/chemical/335|title=Carbon Monoxide - CAMEO Chemicals |publisher=US NOAA Office of Response and Restoration|website=cameochemicals.noaa.gov}}</ref>
|Section6 = {{Chembox Related
|OtherAnions = [[Chemistry:Carbon monosulfide|Carbon monosulfide]]
|OtherCations = [[Chemistry:Silicon monoxide|Silicon monoxide]] <br> [[Chemistry:Germanium monoxide|Germanium monoxide]] <br> [[Chemistry:Tin(II) oxide|Tin(II) oxide]] <br> [[Chemistry:Lead(II) oxide|Lead(II) oxide]]
|OtherFunction_label = carbon oxides
|OtherFunction = [[Chemistry:Carbon dioxide|Carbon dioxide]]<br/>[[Chemistry:Carbon suboxide|Carbon suboxide]]<br/>[[Chemistry:Oxocarbon|Oxocarbon]]s
}}
}}

'''Carbon monoxide''' ([[Chemistry:Chemical formula|chemical formula]] '''CO''') is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one [[Chemistry:Carbon|carbon]] atom and one [[Chemistry:Oxygen|oxygen]] atom connected by a [[Chemistry:Triple bond|triple bond]]. It is the simplest [[Chemistry:Oxocarbon|carbon oxide]]. In [[Chemistry:Coordination complex|coordination complex]]es, the carbon monoxide [[Chemistry:Ligand|ligand]] is called ''[[Chemistry:Metal carbonyl|carbonyl]]''.  It is a key ingredient in many processes in industrial chemistry.<ref name = Ull/>

The most common source of carbon monoxide is the partial [[Physics:Combustion|combustion]] of carbon-containing compounds. Numerous environmental and biological sources generate carbon monoxide. In industry, carbon monoxide is important in the production of many compounds, including drugs, fragrances, and fuels.<ref>{{cite book |doi=10.1002/14356007.a05_203|chapter=Carbon Monoxide|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2001|last1=Bierhals|first1=Jürgen|isbn=3527306730}}</ref> Upon [[Earth:Air pollution|emission]] into the atmosphere, carbon monoxide affects several processes that contribute to [[Earth:Climate change|climate change]].<ref>{{Cite web |last=Voiland |first=Adam |title=Fourteen years of carbon monoxide from MOPITT |url=https://climate.nasa.gov/news/2291/fourteen-years-of-carbon-monoxide-from-mopitt |access-date=2022-03-04 |website=Climate Change: Vital Signs of the Planet}}</ref>

Carbon monoxide has important biological roles across phylogenetic kingdoms. It is produced by many organisms, including humans. In mammalian physiology, carbon monoxide is a classical example of [[Biology:Hormesis|hormesis]] where low concentrations serve as an endogenous [[Biology:Neurotransmitter|neurotransmitter]] ([[Biology:Gasotransmitter|gasotransmitter]]) and high concentrations are toxic resulting in [[Medicine:Carbon monoxide poisoning|carbon monoxide poisoning]]. It is isoelectronic with [[Chemistry:Cyanide|cyanide]] anion CN<sup>−</sup>.

==History==
=== Prehistory ===
Humans have maintained a complex relationship with carbon monoxide since first learning to control fire circa 800,000 BC. Early humans probably discovered the toxicity of carbon monoxide poisoning upon introducing fire into their dwellings. The early development of [[Chemistry:Metallurgy|metallurgy]] and [[Engineering:Smelting|smelting]] technologies emerging circa 6,000 BC through the [[History:Bronze Age|Bronze Age]] likewise plagued humankind from carbon monoxide exposure. Apart from the toxicity of carbon monoxide, indigenous Native Americans may have experienced the neuroactive properties of carbon monoxide through shamanistic fireside rituals.<ref name=":1">{{Cite journal|date=2021-06-01|title=A brief history of carbon monoxide and its therapeutic origins|url=https://www.sciencedirect.com/science/article/abs/pii/S1089860321000367|journal=Nitric Oxide|language=en|volume=111-112|pages=45–63|doi=10.1016/j.niox.2021.04.001|issn=1089-8603|last1=Hopper|first1=Christopher P.|last2=Zambrana|first2=Paige N.|last3=Goebel|first3=Ulrich|last4=Wollborn|first4=Jakob|pmid=33838343|s2cid=233205099}}</ref>

=== Ancient history ===
Early civilizations developed mythological tales to explain the origin of fire, such as [[Religion:Prometheus|Prometheus]] from [[Religion:Greek mythology|Greek mythology]] who shared fire with humans. [[Biography:Aristotle|Aristotle]] (384–322 BC) first recorded that burning coals produced toxic fumes. Greek physician [[Biography:Galen|Galen]] (129–199 AD) speculated that there was a change in the composition of the air that caused harm when inhaled, and many others of the era developed a basis of knowledge about carbon monoxide in the context of [[Earth:Coal|coal]] fume toxicity. Cleopatra may have died from [[Medicine:Carbon monoxide poisoning|carbon monoxide poisoning]].<ref name=":1" />

=== Pre-industrial revolution===
Georg Ernst Stahl mentioned ''carbonarii halitus'' in 1697 in reference to toxic vapors thought to be carbon monoxide. Friedrich Hoffmann conducted the first modern scientific investigation into carbon monoxide poisoning from coal in 1716. Herman Boerhaave conducted the first scientific experiments on the effect of carbon monoxide (coal fumes) on animals in the 1730s.<ref name=":1" />

[[Biography:Joseph Priestley|Joseph Priestley]] is considered to have first synthesized carbon monoxide in 1772. [[Biography:Carl Wilhelm Scheele|Carl Wilhelm Scheele]] similarly isolated carbon monoxide from charcoal in 1773 and thought it could be the carbonic entity making fumes toxic. Torbern Bergman isolated carbon monoxide from [[Chemistry:Oxalic acid|oxalic acid]] in 1775. Later in 1776, the French chemist {{ill|de Lassone|fr|Joseph-Marie-François de Lassone}} produced CO by heating [[Chemistry:Zinc oxide|zinc oxide]] with [[Chemistry:Coke (fuel)|coke]], but mistakenly concluded that the gaseous product was [[Chemistry:Hydrogen|hydrogen]], as it burned with a blue flame. In the presence of oxygen, including atmospheric concentrations, carbon monoxide burns with a blue flame, producing carbon dioxide. [[Biography:Antoine Lavoisier|Antoine Lavoisier]] conducted similar inconclusive experiments to Lassone in 1777. The gas was identified as a compound containing [[Chemistry:Carbon|carbon]] and [[Chemistry:Oxygen|oxygen]] by [[Biography:William Cruickshank (chemist)|William Cruickshank]] in 1800.<ref name=":1" /><ref>{{Cite book|last=Stromeyer|first=Friedrich|url=https://books.google.com/books?id=MLg5AAAAcAAJ&q=hydrocarbonate+cruickshank&pg=PA1|title=Grundriß der theoretischen Chemie: zum Behuf seiner Vorlesungen entworfen|date=1808|publisher=Röwer|pages=1–18|language=de}}</ref>

Thomas Beddoes and James Watt recognized carbon monoxide (as [[Chemistry:Hydrocarbonate (gas)|hydrocarbonate]]) to brighten venous blood in 1793. Watt suggested coal fumes could act as an [[Chemistry:Antidote|antidote]] to the oxygen in blood, and Beddoes and Watt likewise suggested hydrocarbonate has a greater affinity for animal fiber than oxygen in 1796. In 1854, Adrien Chenot similarly suggested carbon monoxide to remove the oxygen from blood and then be oxidized by the body to carbon dioxide.<ref name=":1" /> The mechanism for carbon monoxide poisoning is widely credited to Claude Bernard whose memoirs beginning in 1846 and published in 1857 phrased, "prevents arterials blood from becoming venous". Felix Hoppe-Seyler independently published similar conclusions in the following year.<ref name=":1" />

===Advent of industrial chemistry===
Carbon monoxide gained recognition as an essential reagent in the 1900s.<ref name = Ull>{{Ullmann |doi=10.1002/14356007.a05_203|title=Carbon Monoxide|year=2001|last1=Bierhals|first1=Jürgen|isbn=3527306730}}</ref>  Three industrial processes illustrate its evolution in industry.  In the [[Chemistry:Fischer–Tropsch process|Fischer–Tropsch process]], coal and related carbon-rich feedstocks are converted into liquid fuels via the intermediacy of CO.  Originally developed as part of the German war effort to compensate for their lack of domestic petroleum, this technology continues today.  Also in Germany, a mixture of CO and hydrogen was found to combine with olefins to give [[Chemistry:Aldehyde|aldehyde]]s.  This process, called [[Chemistry:Hydroformylation|hydroformylation]], is used to produce many large scale chemicals such as [[Biology:Surfactant|surfactant]]s as well as specialty compounds that are popular fragrances and drugs.  For example, CO is used in the production of [[Chemistry:Vitamin A|vitamin A]].<ref>{{cite book |doi=10.1016/B978-0-12-801457-8.00014-8|chapter=Carbonylations and Hydroformylations for Fine Chemicals|title=Industrial Catalytic Processes for Fine and Specialty Chemicals|year=2016|last1=Kelkar|first1=A.A.|pages=663–692|isbn=9780128014578}}</ref> In a third major process, attributed to researchers at Monsanto, CO combines with methanol to give [[Chemistry:Acetic acid|acetic acid]]. Most acetic acid is produced by the [[Chemistry:Cativa process|Cativa process]].  Hydroformylation and the acetic acid syntheses are two of myriad [[Chemistry:Carbonylation|carbonylation]] processes.

==Physical and chemical properties==
Carbon monoxide is the simplest [[Chemistry:Oxocarbon|oxocarbon]] and is isoelectronic with other triply-bonded diatomic species possessing 10 valence electrons, including the [[Chemistry:Cyanide|cyanide]] anion, the [[Chemistry:Nitrosonium|nitrosonium]] cation, [[Chemistry:Boron monofluoride|boron monofluoride]] and molecular [[Chemistry:Nitrogen|nitrogen]]. It has a [[Physics:Molar mass|molar mass]] of 28.0, which, according to the [[Physics:Ideal gas law|ideal gas law]], makes it slightly less dense than air, whose average molar mass is 28.8.

The carbon and oxygen are connected by a [[Chemistry:Triple bond|triple bond]] that consists of a net two [[Chemistry:Pi bond|pi bond]]s and one [[Chemistry:Sigma bond|sigma bond]]. The [[Bond length|bond length]] between the carbon atom and the oxygen atom is 112.8&nbsp;[[Picometre|pm]].<ref name="gilliam">{{Cite journal|author1=Gilliam, O. R. |author2=Johnson, C. M. |author3=Gordy, W. |title=Microwave Spectroscopy in the Region from Two to Three Millimeters|year=1950|journal=[[Physics:Physical Review|Physical Review]]|volume=78|issue=2|pages=140–144|doi=10.1103/PhysRev.78.140|bibcode = 1950PhRv...78..140G }}</ref><ref>{{CRC91|page=9–33}}</ref> This bond length is consistent with a triple bond, as in molecular nitrogen (N<sub>2</sub>), which has a similar bond length (109.76&nbsp;pm) and nearly the same [[Physics:Molecular mass|molecular mass]]. Carbon–oxygen double bonds are significantly longer, 120.8&nbsp;pm in [[Physics:Formaldehyde|formaldehyde]], for example.<ref>{{CRC91|page=9–39}}</ref> The boiling point (82&nbsp;K) and melting point (68&nbsp;K) are very similar to those of N<sub>2</sub> (77&nbsp;K and 63&nbsp;K, respectively). The [[Physics:Bond-dissociation energy|bond-dissociation energy]] of 1072&nbsp;kJ/mol is stronger than that of N<sub>2</sub> (942&nbsp;kJ/mol) and represents the strongest chemical bond known.<ref>[http://www.wiredchemist.com/chemistry/data/bond_energies_lengths.html Common Bond Energies (D) and Bond Lengths (r)]. wiredchemist.com</ref>

The [[Physics:Ground state|ground]] electronic state of carbon monoxide is a [[Physics:Singlet state|singlet state]]<ref>{{cite web|title=Highly Excited Triplet States of Carbon Monoxide| first=C. R. |last=Vidal|date=28 June 1997 |url=http://www.mpe.mpg.de/lab/CO/co.html |archive-url=https://web.archive.org/web/20060828201131/http://www.mpe.mpg.de/lab/CO/co.html|archive-date=2006-08-28|access-date= August 16, 2012}}</ref> since there are no unpaired electrons.

{|class="wikitable mw-collapsible"
|+
Thermal and physical properties of carbon monoxide (CO) at atmospheric pressure<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{Cite book |last=Incropera 1 Dewitt 2 Bergman 3 Lavigne 4 |first=Frank P. 1 David P. 2 Theodore L. 3 Adrienne S. 4 |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer. |date=2007 |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755}}</ref>
!Temperature (°C) || Temperature (K) || Density (kg/m<sup>3</sup>) || Specific heat (J/g&nbsp;°C) || Dynamic viscosity (cg/m&nbsp;s) || Kinematic viscosity (cm<sup>2</sup>/s) || Thermal conductivity (cW/m&nbsp;°C) || Thermal diffusivity (cm<sup>2</sup>/s) || [[Physics:Prandtl number|Prandtl number]]
|-
| -73.15
|200||1.6888 ||1.045||1.27||0.0752||1.7 ||0.0963||0.781
|-
| -53.15
|220||1.5341 ||1.044||1.37||0.0893||1.9 ||0.119 ||0.753
|-
| -33.15
|240||1.4055 ||1.043||1.47||0.105 ||2.06||0.141 ||0.744
|-
| -13.15
|260||1.2967 ||1.043||1.57||0.121 ||2.21||0.163 ||0.741
|-
|6.85
|280||1.2038 ||1.042||1.66||0.138 ||2.36||0.188 ||0.733
|-
|26.85
|300||1.1233 ||1.043||1.75||0.156 ||2.5 ||0.213 ||0.73
|-
|46.85
|320||1.0529 ||1.043||1.84||0.175 ||2.63||0.239 ||0.73
|-
|66.85
|340||0.9909 ||1.044||1.93||0.195 ||2.78||0.269 ||0.725
|-
|86.85
|360||0.9357 ||1.045||2.02||0.216 ||2.91||0.298 ||0.725
|-
|106.85
|380||0.8864 ||1.047||2.1 ||0.237 ||3.05||0.329 ||0.729
|-
|126.85
|400||0.8421 ||1.049||2.18||0.259 ||3.18||0.36  ||0.719
|-
|176.85
|450||0.7483 ||1.055||2.37||0.317 ||3.5 ||0.443 ||0.714
|-
|226.85
|500||0.67352||1.065||2.54||0.377 ||3.81||0.531 ||0.71
|-
|276.85
|550||0.61226||1.076||2.71||0.443 ||4.11||0.624 ||0.71
|-
|326.85
|600||0.56126||1.088||2.86||0.51  ||4.4 ||0.721 ||0.707
|-
|376.85
|650||0.51806||1.101||3.01||0.581 ||4.7 ||0.824 ||0.705
|-
|426.85
|700||0.48102||1.114||3.15||0.655 ||5  ||0.933 ||0.702
|-
|476.85
|750||0.44899||1.127||3.29||0.733 ||5.28||1.04  ||0.702
|-
|526.85
|800||0.42095||1.14 ||3.43||0.815 ||5.55||1.16  ||0.705
|}

===Bonding and dipole moment===
Carbon and oxygen together have a total of 10 electrons in the valence shell. Following the [[Octet rule|octet rule]] for both carbon and oxygen, the two atoms form a [[Chemistry:Triple bond|triple bond]], with six shared electrons in three bonding molecular orbitals, rather than the usual double bond found in organic carbonyl compounds. Since four of the shared electrons come from the oxygen atom and only two from carbon, one bonding orbital is occupied by two electrons from oxygen, forming a dative or dipolar bond. This causes a C←O [[Chemistry:Chemical polarity|polarization]] of the molecule, with a small negative charge on carbon and a small positive charge on oxygen. The other two bonding orbitals are each occupied by one electron from carbon and one from oxygen, forming (polar) covalent bonds with a reverse C→O polarization since oxygen is more [[Chemistry:Electronegativity|electronegative]] than carbon. In the free carbon monoxide molecule, a net negative charge δ<sup>–</sup> remains at the carbon end and the molecule has a small [[Chemistry:Bond dipole moment|dipole moment]] of 0.122&nbsp;[[Debye|D]].<ref>{{cite journal | last1=Scuseria | first1=Gustavo E. | last2=Miller | first2=Michael D. | last3=Jensen | first3=Frank | last4=Geertsen | first4=Jan |title=The dipole moment of carbon monoxide|journal=J. Chem. Phys. |volume=94 |issue= 10 | pages=6660 |year=1991 |doi=10.1063/1.460293|bibcode = 1991JChPh..94.6660S }}</ref>

The molecule is therefore asymmetric: oxygen has more electron density than carbon and is also slightly positively charged compared to carbon being negative. By contrast, the isoelectronic dinitrogen molecule has no dipole moment.
[[File:Carbon-monoxide-resonance-2D.png|thumb|The most important resonance form of carbon monoxide is <sup>–</sup>C≡O<sup>+</sup>. An important minor resonance contributor is the non-octet carbenic structure :C=O.]]
Carbon monoxide has a computed fractional bond order of 2.6, indicating that the "third" bond is important but constitutes somewhat less than a full bond.<ref>{{Cite journal|last1=Martinie|first1=Ryan J.|last2=Bultema|first2=Jarred J.|last3=Vander Wal|first3=Mark N.|last4=Burkhart|first4=Brandon J.|last5=Vander Griend|first5=Douglas A.|last6=DeKock|first6=Roger L.|s2cid=11905354|date=2011-08-01|title=Bond Order and Chemical Properties of BF, CO, and N2|journal=Journal of Chemical Education|volume=88|issue=8|pages=1094–1097|doi=10.1021/ed100758t|issn=0021-9584|bibcode=2011JChEd..88.1094M}}</ref>  Thus, in valence bond terms, <sup>–</sup>C≡O<sup>+</sup> is the most important structure, while :C=O is non-octet, but has a neutral formal charge on each atom and represents the second most important resonance contributor.  Because of the lone pair and divalence of carbon in this resonance structure, carbon monoxide is often considered to be an extraordinarily stabilized [[Chemistry:Carbene|carbene]].<ref>{{Cite book|title=Cumulenes in click reactions|last=Ulrich|first=Henri|date=2009|publisher=Wiley|others=Wiley InterScience (Online service)|isbn=9780470747957|location=Chichester, U.K.|pages=45|oclc=476311784}}</ref>  [[Chemistry:Isocyanide|Isocyanide]]s are compounds in which the O is replaced by an NR (R = alkyl or aryl) group and have a similar bonding scheme.

If carbon monoxide acts as a [[Chemistry:Metal carbonyl|ligand]], the polarity of the dipole may reverse with a net negative charge on the oxygen end, depending on the structure of the [[Chemistry:Coordination complex|coordination complex]].<ref>{{cite journal | last1 = Lupinetti | first1 = Anthony J. | last2 = Fau | first2 = Stefan | last3 = Frenking | first3 = Gernot | last4 = Strauss | first4 = Steven H. | title = Theoretical Analysis of the Bonding between CO and Positively Charged Atoms| journal = J. Phys. Chem. A | year= 1997 |volume= 101 |issue=49 |pages= 9551–9559 |doi=10.1021/jp972657l|bibcode= 1997JPCA..101.9551L }}</ref>
See also the section ''"Coordination chemistry"'' below.

===Bond polarity and oxidation state===
Theoretical and experimental studies show that, despite the greater electronegativity of oxygen, the dipole moment points from the more-negative carbon end to the more-positive oxygen end.<ref>{{cite journal | last1 = Blanco | first1 = Fernando | last2 = Alkorta | first2 = Ibon | last3 = Solimannejad | first3 = Mohammad | last4 = Elguero | first4 = Jose | title = Theoretical Study of the 1:1 Complexes between Carbon Monoxide and Hypohalous Acids | journal = J. Phys. Chem. A |year= 2009 |volume= 113 |issue=13 | pmid = 19275137 |pages= 3237–3244 |doi = 10.1021/jp810462h |bibcode = 2009JPCA..113.3237B | hdl = 10261/66300 | hdl-access = free }}</ref><ref>{{cite journal | last1 = Meerts | first1 = W | title = Electric and magnetic properties of carbon monoxide by molecular-beam electric-resonance spectroscopy | journal = Chemical Physics | volume =22 | issue =2 |date= 1 June 1977 | pages =319–324 |doi=10.1016/0301-0104(77)87016-X|bibcode = 1977CP.....22..319M | last2 = De Leeuw | first2 = F.H. | last3 = Dymanus | first3 = A. }}</ref> The three bonds are in fact polar covalent bonds that are strongly polarized. The calculated polarization toward the oxygen atom is 71% for the [[Chemistry:Sigma bond|σ-bond]] and 77% for both [[Chemistry:Pi bond|π-bonds]].<ref name=Stefan>{{cite journal | last1 = Stefan | first1 = Thorsten | last2 = Janoschek | first2 = Rudolf | title = How relevant are S=O and P=O Double Bonds for the Description of the Acid Molecules H<sub>2</sub>SO<sub>3</sub>, H<sub>2</sub>SO<sub>4</sub>, and H<sub>3</sub>PO<sub>3</sub>, respectively? | journal= Journal of Molecular Modeling | volume =6 | issue=2 |year=2000 |pages=282–288 |doi=10.1007/PL00010730| s2cid = 96291857 }}</ref>

The [[Chemistry:Oxidation state|oxidation state]] of carbon in carbon monoxide is +2 in each of these structures. It is calculated by counting all the bonding electrons as belonging to the more electronegative oxygen. Only the two non-bonding electrons on carbon are assigned to carbon. In this count, carbon then has only two valence electrons in the molecule compared to four in the free atom.

==Occurrence==
[[File:MOP CO M.ogv|thumb|Monthly averages of global concentrations of tropospheric carbon monoxide at an altitude of about 12,000 feet. Data were collected by the MOPITT (Measurements Of Pollution In The Troposphere) sensor on NASA's Terra satellite.<ref>[http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MOP_CO_M Global Maps. Carbon Monoxide]. earthobservatory.nasa.gov</ref>]]
Carbon monoxide occurs in various natural and artificial environments. Photochemical degradation of plant matter for example generates an estimated 60 million tons/year.<ref>{{cite journal|doi= 10.1016/S1465-9972(99)00053-7|title= Atmospheric carbon monoxide|year= 1999|last1= Khalil|first1= M.A.K|last2= Pinto|first2= J.P|last3= Shearer|first3= M.J|journal= Chemosphere - Global Change Science|volume= 1|issue= 1–3|pages= ix-xi|bibcode= 1999ChGCS...1D...9K}}</ref> Typical concentrations in [[Parts per million|parts per million]] are as follows:

{| class="wikitable"
|+'''Composition of dry atmosphere, by volume'''<ref>Source for figures: Carbon dioxide, [http://www.esrl.noaa.gov/gmd/ccgg/trends/#mlo NOAA Earth System Research Laboratory], (updated 2010.06). Methane, [[Organization:Intergovernmental Panel on Climate Change|IPCC]] [http://www.grida.no/climate/ipcc_tar/wg1/221.htm#tab61 TAR table 6.1] {{Webarchive|url=https://web.archive.org/web/20070615161122/http://www.grida.no/climate/ipcc_tar/wg1/221.htm#tab61 |date=2007-06-15 }}, (updated to 1998). The NASA total was 17 ppmv over 100%, and {{CO2}} was increased here by 15 ppmv. To normalize, N<sub>2</sub> should be reduced by about 25 ppmv and O<sub>2</sub> by about 7 ppmv.</ref>
|-
! Concentration (ppmv{{efn|[[Parts per million]] by volume (note: [[Chemistry:Volume fraction|volume fraction]] is equal to [[Chemistry:Mole fraction|mole fraction]] for ideal gas only, see [[Physics:Volume (thermodynamics)#Partial volume|volume (thermodynamics)]])}})
! Source
|-
| 0.1
| Natural atmosphere level ([[Engineering:MOPITT|MOPITT]])<ref>{{Cite book|author=Committee on Medical and Biological Effects of Environmental Pollutants|title=Carbon Monoxide|publisher=National Academy of Sciences|year=1977|isbn=978-0-309-02631-4|location=Washington, D.C.|pages=29}}</ref>
|-
| 0.5–5
| Average level in homes<ref name="EPAFAQ">{{Cite web|author=Green W|title=An Introduction to Indoor Air Quality: Carbon Monoxide (CO)|url=http://www.epa.gov/iaq/co.html|access-date=2008-12-16|publisher=United States Environmental Protection Agency}}</ref>
|-
| 5–15
| Near properly-adjusted gas stoves in homes, modern vehicle exhaust emissions<ref name="Alaska_CO">{{Cite web|last=Gosink|first=Tom|date=1983-01-28|title=What Do Carbon Monoxide Levels Mean?|url=http://www.gi.alaska.edu/ScienceForum/ASF5/588.html|url-status=dead|archive-url=https://web.archive.org/web/20081225113654/http://www.gi.alaska.edu/ScienceForum/ASF5/588.html|archive-date=2008-12-25|access-date=2007-12-01|work=Alaska Science Forum|publisher=Geophysical Institute, University of Alaska Fairbanks}}</ref>{{Citation needed|date=May 2020}}
|-
| 17
| [[Astronomy:Atmosphere of Venus|Atmosphere of Venus]]
|-
| 100–200
| Exhaust from automobiles in the Mexico City central area in 1975<ref>{{Cite book|last=Singer|first=Siegfried Fred|title=The Changing Global Environment|publisher=Springer|year=1975|isbn=978-9027704023|page=90}}</ref>
|-
| 700
| [[Astronomy:Atmosphere of Mars|Atmosphere of Mars]]
|-
| <1,000
| Car exhaust fumes after passing through [[Chemistry:Catalytic converter|catalytic converter]]<ref name="Iowa_CO">{{cite web|title=Carbon Monoxide Poisoning: Vehicles (AEN-208)|url=https://www.abe.iastate.edu/extension-and-outreach/carbon-monoxide-poisoning-vehicles-aen-208/|access-date=11 February 2018|website=abe.iastate.edu}}</ref>
|-
| 5,000
| Exhaust from a home wood fire<ref name="Gosink">{{Cite web|author=Gosink T|date=January 28, 1983|title=What Do Carbon Monoxide Levels Mean?|url=http://www.gi.alaska.edu/ScienceForum/ASF5/588.html|url-status=dead|archive-url=https://web.archive.org/web/20081225113654/http://www.gi.alaska.edu/ScienceForum/ASF5/588.html|archive-date=December 25, 2008|access-date=December 16, 2008|work=Alaska Science Forum|publisher=Geophysical Institute, University of Alaska Fairbanks}}</ref>
|-
| 30,000–100,000
| Undiluted warm car exhaust without a [[Chemistry:Catalytic converter|catalytic converter]]<ref name="Iowa_CO" />
|-
| colspan="2" style="font-size: 85%" | {{noteslist}}
|}

===Atmospheric presence===
[[File:Carbon Monoxide.ogv|thumb|The streak of red, orange, and yellow across [[Earth:South America|South America]], Africa, and the Atlantic Ocean in this animation points to high levels of carbon monoxide on September 30, 2005.]]
[[File:Carbon Monoxide concentrations in spring..jpg|thumb|Carbon monoxide concentrations in Northern Hemisphere spring as measured with the MOPITT instrument]]

Carbon monoxide (CO) is present in small amounts (about 80 ppb) in the [[Earth:Atmosphere of Earth|Earth's atmosphere]]. Most of the rest comes from chemical reactions with [[Chemistry:Organic compound|organic compound]]s emitted by human activities and natural origins due to [[Chemistry:Photochemistry|photochemical]] reactions in the [[Earth:Troposphere|troposphere]] that generate about 5 × 10<sup>12</sup> kilograms per year.<ref name=":0">{{Cite journal|last1=Weinstock|first1=B.|last2=Niki|first2=H.|year=1972|title=Carbon Monoxide Balance in Nature|journal=Science|volume=176|issue=4032|pages=290–2|bibcode=1972Sci...176..290W|doi=10.1126/science.176.4032.290|pmid=5019781|s2cid=25223868}}</ref> Other natural sources of CO include volcanoes, forest and bushfires, and other miscellaneous forms of combustion such as fossil fuels.<ref name="Seinfeld2006">{{cite book|last1=Seinfeld|first1=John|title=Atmospheric Chemistry and Physics: From Air Pollution to Climate Change|last2=Pandis|first2=Spyros|date=2006|publisher=John Wiley & Sons|isbn=978-0-471-72018-8}}</ref> Small amounts are also emitted from the ocean, and from geological activity because carbon monoxide occurs dissolved in molten volcanic rock at high [[Physics:Pressure|pressure]]s in the Earth's [[Earth:Mantle (geology)|mantle]].<ref>{{Cite book|author1=Sigel, Astrid|url=https://books.google.com/books?id=kcllhZcy53cC&pg=PA243|title=Metal-Carbon Bonds in Enzymes and Cofactors|author2=Sigel, Roland K. O.|publisher=Royal Society of Chemistry|year=2009|isbn=978-1-84755-915-9|page=243}}</ref> Because natural sources of carbon monoxide vary from year to year, it is difficult to accurately measure natural emissions of the gas.

Carbon monoxide has an indirect effect on [[Astronomy:Radiative forcing|radiative forcing]] by elevating concentrations of direct [[Chemistry:Greenhouse gas|greenhouse gas]]es, including [[Chemistry:Methane|methane]] and [[Earth:Troposphere|tropospheric]] [[Chemistry:Ozone|ozone]]. CO can react chemically with other atmospheric constituents (primarily the hydroxyl [[Biology:Radical|radical]], <sup>•</sup>OH) that would otherwise destroy methane.<ref>{{Cite book|author=White, James Carrick|url=https://books.google.com/books?id=gnaNHeU4rgQC&pg=PA106|title=Global climate change linkages: acid rain, air quality, and stratospheric ozone|publisher=Springer|year=1989|isbn=978-0-444-01515-0|page=106|display-authors=0}}</ref> Through natural processes in the atmosphere, it is oxidized to [[Chemistry:Carbon dioxide|carbon dioxide]] and ozone. Carbon monoxide is short-lived in the atmosphere (with an average lifetime of about one to two months), and spatially variable in concentration.<ref>{{cite web|last=Drummond|first=James|date=February 2, 2018|title=MOPITT, Atmospheric Pollution, and Me: A Personal Story|url=http://bulletin.cmos.ca/mopitt-atmospheric-pollution/|access-date=August 1, 2018|publisher=Canadian Meteorological and Oceanographic Society}}</ref>

Due to its long lifetime in the mid-troposphere, carbon monoxide is also used as a tracer for pollutant plumes.<ref>{{cite journal|last1=Pommier|first1=M.|last2=Law|first2=K. S.|last3=Clerbaux|first3=C.|last4=Turquety|first4=S.|last5=Hurtmans|first5=D.|last6=Hadji-Lazaro|first6=J.|last7=Coheur|first7=P.-F.|last8=Schlager|first8=H.|last9=Ancellet|first9=G.|last10=Paris|first10=J.-D.|last11=Nédélec|first11=P.|year=2010|title=IASI carbon monoxide validation over the Arctic during POLARCAT spring and summer campaigns|url=https://hal.archives-ouvertes.fr/hal-00491181|journal=Atmospheric Chemistry and Physics|volume=10|issue=21|pages=10655–10678|bibcode=2010ACP....1010655P|doi=10.5194/acp-10-10655-2010|doi-access=free|last15=Bernath|first15=P.|first14=J. S.|last14=Holloway|first13=J. R.|last13=Podolske|first12=G. S.|last12=Diskin}}</ref>

==== Pollution ====
{{Main|Earth:Air pollution}}

=====Urban pollution=====
Carbon monoxide is a temporary atmospheric pollutant in some urban areas, chiefly from the exhaust of internal combustion engines (including vehicles, portable and back-up generators, lawnmowers, power washers, etc.), but also from incomplete combustion of various other fuels (including wood, coal, charcoal, oil, paraffin, propane, natural gas, and trash).

Large CO pollution events can be observed from space over cities.<ref>{{Cite journal|last1=Pommier|first1=M.|last2=McLinden|first2=C. A.|last3=Deeter|first3=M.|year=2013|title=Relative changes in CO emissions over megacities based on observations from space|url=https://hal.archives-ouvertes.fr/hal-00844309/document|journal=Geophysical Research Letters|volume=40|issue=14|pages=3766|bibcode=2013GeoRL..40.3766P|doi=10.1002/grl.50704|doi-access=free}}</ref>

======Role in ground level ozone formation======
{{Main|Earth:Ground level ozone}}
Carbon monoxide is, along with [[Chemistry:Aldehyde|aldehyde]]s, part of the series of cycles of chemical reactions that form photochemical smog. It reacts with hydroxyl radical (<sup>•</sup>OH) to produce a radical intermediate <sup>•</sup>HOCO, which transfers rapidly its radical hydrogen to O<sub>2</sub> to form peroxy radical (HO<sub>2</sub><sup>•</sup>) and carbon dioxide ({{CO2}}).<ref>{{Cite journal|author1=Reeves, Claire E.|author2=Penkett, Stuart A.|author3=Bauguitte, Stephane|author4=Law, Kathy S.|author5=Evans, Mathew J.|author6=Bandy, Brian J.|author7=Monks, Paul S.|author8=Edwards, Gavin D.|author9=Phillips, Gavin|author10=Barjat, Hannah|author11=Kent, Joss|year=2002|title=Potential for photochemical ozone formation in the troposphere over the North Atlantic as derived from aircraft observationsduring ACSOE|url=http://juser.fz-juelich.de/record/34366|journal=Journal of Geophysical Research|volume=107|issue=D23|pages=4707|bibcode=2002JGRD..107.4707R|doi=10.1029/2002JD002415|doi-access=free|author14=Kley, Dieter|author13=Schmitgen, Sandra|author12=Dewey, Ken}}</ref> Peroxy radical subsequently reacts with [[Chemistry:Nitrogen oxide|nitrogen oxide]] (NO) to form [[Chemistry:Nitrogen dioxide|nitrogen dioxide]] (NO<sub>2</sub>) and hydroxyl radical. NO<sub>2</sub> gives O(<sup>3</sup>P) via photolysis, thereby forming O<sub>3</sub> following reaction with O<sub>2</sub>.
Since hydroxyl radical is formed during the formation of NO<sub>2</sub>, the balance of the sequence of chemical reactions starting with carbon monoxide and leading to the formation of ozone is:
:CO + 2O<sub>2</sub> + hν → {{CO2}} + O<sub>3</sub>
(where hν refers to the [[Physics:Photon|photon]] of light absorbed by the NO<sub>2</sub> molecule in the sequence)

Although the creation of NO<sub>2</sub> is the critical step leading to low level [[Chemistry:Ozone|ozone]] formation, it also increases this ozone in another, somewhat mutually exclusive way, by reducing the quantity of NO that is available to react with ozone.<ref name="NA">{{Cite book|url=https://books.google.com/books?id=QkErAAAAYAAJ&pg=PA23|title=Ozone and other photochemical oxidants|publisher=National Academies|year=1977|isbn=978-0-309-02531-7|page=23}}</ref>

=====Indoor pollution=====
In closed environments, the concentration of carbon monoxide can rise to lethal levels. On average, 170 people in the United States die every year from carbon monoxide produced by non-automotive consumer products.<ref name="cpsc">
  [https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center/Carbon-Monoxide-Questions-and-Answers Carbon Monoxide Questions and Answers] from
  U.S. Consumer Product Safety Commission.
  Retrieved 2023-02-20.
  {{
     webarchive
       | url=https://web.archive.org/web/20230117052704/https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center/Carbon-Monoxide-Questions-and-Answers
       | date=2023-01-17
       | title=Archive copy
  }}.
</ref>
These products include malfunctioning fuel-burning appliances such as furnaces, ranges, water heaters, and gas and [[Engineering:Kerosene heater|kerosene]] room heaters; engine-powered equipment such as portable generators (and cars left running in attached garages); fireplaces; and charcoal that is burned in homes and other enclosed areas. Many deaths have occurred during power outages due to severe weather such as Hurricane Katrina<ref name="cpsc" /> and the 2021 Texas power crisis.<ref>{{Cite news|title='A Disaster Within A Disaster': Carbon Monoxide Poisoning Cases Are Surging In Texas|url=https://www.npr.org/sections/live-updates-winter-storms-2021/2021/02/18/969050038/a-disaster-within-a-disaster-carbon-monoxide-poisoning-cases-are-surging-in-texa|access-date=2021-05-16|newspaper=NPR|date=18 February 2021|language=en|last1=Treisman|first1=Rachel}}</ref>

=== Mining ===
Miners refer to carbon monoxide as "whitedamp" or the "silent killer". It can be found in confined areas of poor ventilation in both surface mines and underground mines. The most common sources of carbon monoxide in mining operations are the internal combustion engine and explosives; however, in coal mines, carbon monoxide can also be found due to the low-temperature oxidation of coal.<ref>{{Cite web|title=MSHA - Occupational Illness and Injury Prevention Program - Health Topics - Carbon Monoxide|url=https://arlweb.msha.gov/illness_prevention/healthtopics/carbonmonoxide.htm|url-status=dead|archive-url=https://web.archive.org/web/20171231212725/https://arlweb.msha.gov/illness_prevention/healthtopics/carbonmonoxide.htm|archive-date=2017-12-31|access-date=2017-12-31|website=arlweb.msha.gov|language=en}}</ref> The idiom "Canary in the coal mine" pertained to an early warning of a carbon monoxide presence.<ref name=":1" />

=== Astronomy ===
Beyond Earth, carbon monoxide is the second-most common diatomic molecule in the [[Astronomy:Interstellar medium|interstellar medium]], after molecular hydrogen.  Because of its asymmetry, this polar molecule produces far brighter [[Physics:Spectral line|spectral line]]s than the hydrogen molecule, making CO much easier to detect.  Interstellar CO was first detected with [[Astronomy:Radio telescope|radio telescope]]s in 1970. It is now the most commonly used tracer of molecular gas in general in the interstellar medium of galaxies, as molecular hydrogen can only be detected using ultraviolet light, which requires space telescopes. Carbon monoxide observations provide much of the information about the [[Astronomy:Molecular cloud|molecular cloud]]s in which most [[Astronomy:Star formation|stars form]].<ref>{{cite journal|author=Combes, Françoise|year=1991|title=Distribution of CO in the Milky Way|journal=Annual Review of Astronomy & Astrophysics|volume=29|pages=195–237|bibcode=1991ARA&A..29..195C|doi=10.1146/annurev.aa.29.090191.001211}}</ref><ref>{{cite journal|author=Hamed, M.|year=2021|title=Multiwavelength dissection of a massive heavily dust-obscured galaxy and its blue companion at z~2|journal=Astronomy & Astrophysics|volume=646|pages=A127|bibcode=2021A&A...646A.127H|doi=10.1051/0004-6361/202039577|arxiv=2101.07724|s2cid=231639096}}</ref>

[[Astronomy:Beta Pictoris|Beta Pictoris]], the second brightest [[Astronomy:Star|star]] in the [[Astronomy:Constellation|constellation]] [[Astronomy:Pictor|Pictor]], shows an [[Astronomy:Infrared excess|excess of infrared emission]] compared to normal stars of its type, which is caused by large quantities of dust and gas (including carbon monoxide)<ref name="LAT-20140308">{{cite news|last=Khan|first=Amina|title=Did two planets around nearby star collide? Toxic gas holds hints|work={{wipe|Los Angeles Times}}|url=http://www.latimes.com/science/sciencenow/la-sci-sn-beta-pictoris-star-planet-gas-collision-comets-carbon-monoxide-20140307,0,1884709.story|access-date=March 9, 2014}}</ref><ref name="SCI-20140306">{{cite journal|vauthors=Dent, WR, Wyatt, MC, Roberge, A, Augereau, JC, Casassus, S, Corder, S, Greaves, JS, ((de Gregorio-Monsalvo, I)), Hales, A, Jackson, AP, Hughes, AM, Meredith A, Lagrange, AM, Matthews, B, Wilner, D|date=March 6, 2014|title=Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk|url=https://www.science.org/doi/abs/10.1126/science.1248726|journal=Science|volume=343|issue=6178|pages=1490–1492|arxiv=1404.1380|bibcode=2014Sci...343.1490D|doi=10.1126/science.1248726|pmid=24603151|access-date=March 9, 2014|s2cid=206553853}}</ref> near the star.

In the [[Astronomy:Atmosphere of Venus|atmosphere of Venus]] carbon monoxide occurs as a result of the photodissociation of carbon dioxide by electromagnetic radiation of wavelengths shorter than 169 [[Nanometre|nm]]. It has also been identified spectroscopically on the surface of Neptune's moon [[Astronomy:Triton (moon)|Triton]].<ref name="Lellouchde Bergh2010">{{cite journal|last1=Lellouch|first1=E.|last2=de Bergh|first2=C.|last3=Sicardy|first3=B.|last4=Ferron|first4=S.|last5=Käufl|first5=H.-U.|year=2010|title=Detection of CO in Triton's atmosphere and the nature of surface-atmosphere interactions|journal=Astronomy and Astrophysics|volume=512|pages=L8|arxiv=1003.2866|bibcode=2010A&A...512L...8L|doi=10.1051/0004-6361/201014339|issn=0004-6361|s2cid=58889896}}</ref>

Solid carbon monoxide is a component of [[Astronomy:Comet|comet]]s.<ref>{{cite journal|last1=Greenberg|first1=J. Mayo|date=1998|title=Making a comet nucleus|journal=Astronomy and Astrophysics|volume=330|pages=375|bibcode=1998A&A...330..375G}}</ref> The [[Earth:Volatile (astrogeology)|volatile or "ice"]] component of [[Astronomy:Halley's Comet|Halley's Comet]] is about 15% CO.<ref name="Yeomans">{{cite web|last=Yeomans|first=Donald K.|date=2005|title=Comets (World Book Online Reference Center 125580)|url=http://www.nasa.gov/worldbook/comet_worldbook.html|url-status=dead|archive-url=https://web.archive.org/web/20050429084558/http://www.nasa.gov/worldbook/comet_worldbook.html|archive-date=29 April 2005|access-date=18 August 2022|quote=About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent.|publisher=NASA|df=dmy-all}}</ref> At room temperature and at atmospheric pressure, carbon monoxide is actually only metastable (see [[Physics:Boudouard reaction|Boudouard reaction]]) and the same is true at low temperatures where CO and {{chem|CO|2}} are solid, but nevertheless it can exist for billions of years in comets. There is very little CO in the atmosphere of [[Astronomy:Pluto|Pluto]], which seems to have been formed from comets. This may be because there is (or was) liquid water inside Pluto.

Carbon monoxide can react with water to form carbon dioxide and hydrogen:

:CO + H<sub>2</sub>O → {{chem|H|2}} + {{chem|CO|2}}

This is called the [[Chemistry:Water-gas shift reaction|water-gas shift reaction]] when occurring in the gas phase, but it can also take place (very slowly) in an aqueous solution.
If the hydrogen partial pressure is high enough (for instance in an underground sea), [[Chemistry:Formic acid|formic acid]] will be formed:

:CO + H<sub>2</sub>O → HCOOH

These reactions can take place in a few million years even at temperatures such as found on Pluto.<ref>{{cite journal|last1=Christopher Glein and Hunter Waite|date=May 11, 2018|title=Primordial N2 provides a cosmo chemical explanation for the existence of Sputnik Planitia, Pluto|journal=Icarus|volume=313|pages=79–92|arxiv=1805.09285|bibcode=2018Icar..313...79G|doi=10.1016/j.icarus.2018.05.007|s2cid=102343522}}</ref>

== Chemistry ==
Carbon monoxide has a wide range of functions across all disciplines of chemistry. The four premier categories of reactivity involve [[Chemistry:Metal carbonyl|metal-carbonyl]] catalysis, [[Biology:Radical|radical]] chemistry, cation and anion chemistries.<ref>{{Cite journal|date=2019-03-14|title=The Chemistry of CO: Carbonylation|journal=Chem|language=en|volume=5|issue=3|pages=526–552|doi=10.1016/j.chempr.2018.11.006|issn=2451-9294|doi-access=free|last1=Peng|first1=Jin-Bao|last2=Geng|first2=Hui-Qing|last3=Wu|first3=Xiao-Feng}}</ref>

===Coordination chemistry===
{{Main|Chemistry:Metal carbonyl}}

[[File:MO COeng.svg|thumb|left|200px|Energy level scheme of the σ and π orbitals of carbon monoxide]]
[[Image:Carbon-monoxide-HOMO-phase-3D-balls.svg|thumb|right|150px|The HOMO of CO is a σ MO.]]
[[Image:Carbon-monoxide-LUMO-phase-3D-balls.png|thumb|right|150px|The LUMO of CO is a π* antibonding MO.]]

Most metals form [[Chemistry:Coordination complex|coordination complex]]es containing covalently attached carbon monoxide. Only metals in lower oxidation states will complex with carbon monoxide ligands. This is because there must be sufficient electron density to facilitate back-donation from the metal d<sub>xz</sub>-orbital, to the π* [[Chemistry:Molecular orbital|molecular orbital]] from CO. The lone pair on the carbon atom in CO also donates electron density to the d<sub>x<sup>2</sup>−y<sup>2</sup></sub> on the metal to form a [[Chemistry:Sigma bond|sigma bond]]. This electron donation is also exhibited with the [[Chemistry:Cis effect|cis effect]], or the labilization of CO ligands in the cis position. Nickel carbonyl, for example, forms by the direct combination of carbon monoxide and [[Chemistry:Nickel|nickel]] metal:
: Ni + 4 CO → Ni(CO)<sub>4</sub> (1 [[Bar (unit)|bar]], 55 °C)
For this reason, nickel in any tubing or part must not come into prolonged contact with carbon monoxide. Nickel carbonyl decomposes readily back to Ni and CO upon contact with hot surfaces, and this method is used for the industrial purification of nickel in the [[Chemistry:Mond process|Mond process]].<ref>{{Cite journal|vauthors=Mond L, Langer K, Quincke F|year=1890|title=Action of carbon monoxide on nickel|url=https://zenodo.org/record/2160347|journal=Journal of the Chemical Society|volume=57|pages=749–753|doi=10.1039/CT8905700749}}</ref>

In nickel carbonyl and other carbonyls, the electron pair on the carbon interacts with the metal; the carbon monoxide donates the electron pair to the metal. In these situations, carbon monoxide is called the '''carbonyl''' [[Chemistry:Ligand|ligand]]. One of the most important metal carbonyls is [[Chemistry:Iron pentacarbonyl|iron pentacarbonyl]], Fe(CO)<sub>5</sub>:

[[Image:IronPentacarbonylStructure.png|150px|Structure of iron pentacarbonyl.]]
[[Image:Iron-pentacarbonyl-3D-balls.png|120px|Iron pentacarbonyl.]]

Many metal–CO complexes are prepared by decarbonylation of organic solvents, not from CO. For instance, [[Chemistry:Iridium(III) chloride|iridium trichloride]] and [[Chemistry:Triphenylphosphine|triphenylphosphine]] react in boiling [[Chemistry:2-Methoxyethanol|2-methoxyethanol]] or [[Chemistry:Dimethylformamide|DMF]] to afford [[Chemistry:Vaska's complex|IrCl(CO)(PPh<SUB>3</SUB>)<SUB>2</SUB>]].

Metal carbonyls in coordination chemistry are usually studied using infrared spectroscopy.

===Organic and main group chemistry===
{{Main|Chemistry:Carbonylation}}
In the presence of strong acids and water, carbon monoxide reacts with [[Chemistry:Alkene|alkene]]s to form carboxylic acids in a process known as the Koch–Haaf reaction.<ref name="koch">{{OrgSynth|author=Koch, H.|author2=Haaf, W.|title=1-Adamantanecarboxylic Acid|year=1973|collvol=5|collvolpages=20|prep=cv5p0020}}</ref> In the Gattermann–Koch reaction, [[Chemistry:Aromatic hydrocarbon|arenes]] are converted to [[Chemistry:Benzaldehyde|benzaldehyde]] derivatives in the presence of CO, [[Chemistry:Aluminium chloride|AlCl<sub>3</sub>]] and [[Chemistry:Hydrogen chloride|HCl]].<ref name="coleman">{{OrgSynth|title=''p''-Tolualdehyde|author=Coleman, G. H.|author2=Craig, David|collvol=2|collvolpages=583|year=1943|prep=cv2p0583}}</ref> Organolithium compounds (e.g. butyl lithium) react with carbon monoxide, but these reactions have little scientific use.

Although CO reacts with [[Chemistry:Carbocation|carbocation]]s and [[Chemistry:Carbanion|carbanion]]s, it is relatively nonreactive toward organic compounds without the intervention of metal catalysts.<ref>Chatani, N.; Murai, S. "Carbon Monoxide" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. {{doi|10.1002/047084289X}}</ref>

With main group reagents, CO undergoes several noteworthy reactions. [[Chemistry:Halogenation|Chlorination]] of CO is the industrial route to the important compound [[Chemistry:Phosgene|phosgene]]. With [[Chemistry:Borane|borane]] CO forms the adduct [[Chemistry:Borane carbonyl|H<sub>3</sub>BCO]], which is isoelectronic with the acetylium cation [H<sub>3</sub>CCO]<sup>+</sup>. CO reacts with [[Chemistry:Sodium|sodium]] to give products resulting from C−C coupling such as [[Chemistry:Acetylenediol|sodium acetylenediolate]] 2{{chem|Na|+}}·{{chem|C|2|O|2|2-}}. It reacts with molten [[Chemistry:Potassium|potassium]] to give a mixture of an organometallic compound, [[Chemistry:Acetylenediol|potassium acetylenediolate]] 2{{chem|K|+}}·{{chem|C|2|O|2|2-}}, [[Chemistry:Benzenehexol|potassium benzenehexolate]] 6{{chem|K|+}}{{chem|C|6|O|6|6-}},<ref name="wbuechIV">{{cite journal|last1=Büchner|first1=W.|last2=Weiss|first2=E.|year=1964|title=Zur Kenntnis der sogenannten "Alkalicarbonyle" IV[1] Über die Reaktion von geschmolzenem Kalium mit Kohlenmonoxid|journal=Helvetica Chimica Acta|volume=47|issue=6|pages=1415–1423|doi=10.1002/hlca.19640470604}}
</ref> and [[Chemistry:Rhodizonic acid|potassium rhodizonate]] 2{{chem|K|+}}·{{chem|C|6|O|6|2-}}.<ref>{{Cite book|author=Fownes, George|url=https://archive.org/details/amanualelementa02fowngoog|title=A Manual of elementary chemistry|publisher=H.C. Lea|year=1869|page=[https://archive.org/details/amanualelementa02fowngoog/page/n682 678]}}</ref>

The compounds [[Chemistry:Cyclohexanehexone|cyclohexanehexone]] or triquinoyl (C<sub>6</sub>O<sub>6</sub>) and [[Chemistry:Cyclopentanepentone|cyclopentanepentone]] or leuconic acid (C<sub>5</sub>O<sub>5</sub>), which so far have been obtained only in trace amounts, can be regarded as polymers of carbon monoxide. At pressures exceeding 5 [[Pascal (unit)|GPa]], carbon monoxide converts to [[Chemistry:Polycarbonyl|polycarbonyl]], a solid polymer that is metastable at atmospheric pressure but is explosive.<ref>{{Cite journal|last1=Katz|first1=Allen I.|last2=Schiferl|first2=David|last3=Mills|first3=Robert L.|year=1984|title=New phases and chemical reactions in solid carbon monoxide under pressure|journal=The Journal of Physical Chemistry|volume=88|issue=15|pages=3176–3179|doi=10.1021/j150659a007}}</ref><ref>{{Cite journal|last1=Evans|first1=W. J.|last2=Lipp|first2=M. J.|last3=Yoo|first3=C.-S.|last4=Cynn|first4=H.|last5=Herberg|first5=J. L.|last6=Maxwell|first6=R. S.|last7=Nicol|first7=M. F.|year=2006|title=Pressure-Induced Polymerization of Carbon Monoxide: Disproportionation and Synthesis of an Energetic Lactonic Polymer|url=https://digital.library.unt.edu/ark:/67531/metadc892676/|journal=Chemistry of Materials|volume=18|issue=10|pages=2520–2531|doi=10.1021/cm0524446}}</ref>

==== Laboratory preparation ====
Carbon monoxide is conveniently produced in the laboratory by the [[Chemistry:Dehydration reaction|dehydration]] of [[Chemistry:Formic acid|formic acid]] or [[Chemistry:Oxalic acid|oxalic acid]], for example with concentrated [[Chemistry:Sulfuric acid|sulfuric acid]].<ref name="koch" /><ref name="coleman" /><ref name="Georg">{{cite book|last=Brauer|first=Georg|url=https://books.google.com/books?id=TLYatwAACAAJ&q=Handbook+of+Preparative+Inorganic+Chemistry|title=Handbook of Preparative Inorganic Chemistry Vol. 1, 2nd Ed|date=1963|publisher=Academic Press|isbn=978-0121266011|location=New York|page=646}}</ref> Another method is heating an intimate mixture of powdered [[Chemistry:Zinc|zinc]] metal and [[Chemistry:Calcium carbonate|calcium carbonate]], which releases CO and leaves behind [[Chemistry:Zinc oxide|zinc oxide]] and [[Chemistry:Calcium oxide|calcium oxide]]:

:Zn + CaCO<sub>3</sub> → ZnO + CaO + CO

[[Chemistry:Silver nitrate|Silver nitrate]] and [[Chemistry:Iodoform|iodoform]] also afford carbon monoxide:

:CHI<sub>3</sub> + 3AgNO<sub>3</sub> + H<sub>2</sub>O → 3HNO<sub>3</sub> + CO + 3AgI

Finally, metal [[Chemistry:Oxalate|oxalate]] salts release CO upon heating, leaving a [[Chemistry:Carbonate|carbonate]] as byproduct:

: {{chem|Na|2|C|2|O|4}} → {{chem|Na|2|C|O|3}} + {{chem|CO}}

== Production ==
Thermal [[Physics:Combustion|combustion]] is the most common source for carbon monoxide. Carbon monoxide is produced from the partial oxidation of [[Chemistry:Carbon|carbon]]-containing compounds; it forms when there is not enough oxygen to produce [[Chemistry:Carbon dioxide|carbon dioxide]] ({{CO2}}), such as when operating a [[Engineering:Stove|stove]] or an [[Earth:Internal combustion engine|internal combustion engine]] in an enclosed space.

A large quantity of CO byproduct is formed during the oxidative processes for the production of chemicals. For this reason, the process off-gases have to be purified. 

Many methods have been developed for carbon monoxide production.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 200. {{ISBN|0-12-352651-5}}.</ref>

===Industrial production===
A major industrial source of CO is [[Chemistry:Producer gas|producer gas]], a mixture containing mostly carbon monoxide and nitrogen, formed by combustion of carbon in air at high temperature when there is an excess of carbon. In an oven, air is passed through a bed of [[Chemistry:Coke (fuel)|coke]]. The initially produced {{CO2}} equilibrates with the remaining hot carbon to give CO.<ref>{{Cite web|title=Carbon Monoxide|url=http://www.chm.bris.ac.uk/motm/co/coh.htm|access-date=21 May 2021}}</ref> The reaction of {{CO2}} with carbon to give CO is described as the [[Physics:Boudouard reaction|Boudouard reaction]].<ref>{{cite book|author1=Higman, C|url=https://archive.org/details/gasification00higm|title=Gasification|author2=van der Burgt, M|publisher=Gulf Professional Publishing|year=2003|isbn=978-0-7506-7707-3|page=[https://archive.org/details/gasification00higm/page/n22 12]|url-access=limited}}</ref> Above 800&nbsp;°C, CO is the predominant product:

:{{CO2}} (g) + C (s) → 2 CO (g) (Δ''H''<sub>r</sub> = 170 kJ/mol)

Another source is "[[Chemistry:Water gas|water gas]]", a mixture of [[Chemistry:Hydrogen|hydrogen]] and carbon monoxide produced via the endothermic reaction of [[Physics:Steam|steam]] and carbon:

:H<sub>2</sub>O (g) + C (s) → H<sub>2</sub> (g) + CO (g) (Δ''H''<sub>r</sub> = 131 kJ/mol)

Other similar "[[Chemistry:Syngas|synthesis gas]]es" can be obtained from [[Chemistry:Natural gas|natural gas]] and other fuels.

Carbon monoxide can also be produced by [[Physics:High-temperature electrolysis|high-temperature electrolysis]] of carbon dioxide with solid oxide electrolyzer cells.<ref>{{cite journal|last1=Zheng|first1=Yun|last2=Wang|first2=Jianchen|last3=Yu|first3=Bo|last4=Zhang|first4=Wenqiang|last5=Chen|first5=Jing|last6=Qiao|first6=Jinli|last7=Zhang|first7=Jiujun|date=2017|title=A review of high temperature co-electrolysis of H O and CO to produce sustainable fuels using solid oxide electrolysis cells (SOECs): advanced materials and technology|journal=Chem. Soc. Rev.|volume=46|issue=5|pages=1427–1463|doi=10.1039/C6CS00403B|pmid=28165079}}</ref> One method developed at DTU Energy uses a cerium oxide catalyst and does not have any issues of fouling of the catalyst.<ref>{{Cite web|title=New route to carbon-neutral fuels from carbon dioxide discovered by Stanford-DTU team - DTU|url=https://www.dtu.dk/english/news/2019/09/new-route-to-carbon-neutral-fuels-from-carbon-dioxide-discovered-by-stanford-dtu-team?id=4feafcd3-bf1d-4e02-80ea-ea018bfc6caf|website=dtu.dk}}</ref><ref>{{Cite journal|last1=Skafte|first1=Theis L.|last2=Guan|first2=Zixuan|last3=Machala|first3=Michael L.|last4=Gopal|first4=Chirranjeevi B.|last5=Monti|first5=Matteo|last6=Martinez|first6=Lev|last7=Stamate|first7=Eugen|last8=Sanna|first8=Simone|last9=Garrido Torres|first9=Jose A.|last10=Crumlin|first10=Ethan J.|last11=García-Melchor|first11=Max|date=October 8, 2019|title=Selective high-temperature CO 2 electrolysis enabled by oxidized carbon intermediates|url=https://www.nature.com/articles/s41560-019-0457-4|journal=Nature Energy|volume=4|issue=10|pages=846–855|doi=10.1038/s41560-019-0457-4|via=www.nature.com|last12=Bajdich|first13=William C.|last13=Chueh|first14=Christopher|last14=Graves|first12=Michal|bibcode=2019NatEn...4..846S|hdl=2262/93685|s2cid=202640892|hdl-access=free}}</ref>

:2 {{CO2}} → 2 CO + O<sub>2</sub>

Carbon monoxide is also a byproduct of the reduction of metal [[Chemistry:Oxide|oxide]] [[Earth:Ore|ore]]s with carbon, shown in a simplified form as follows:

:MO + C → M + CO

Carbon monoxide is also produced by the direct oxidation of carbon in a limited supply of oxygen or air.

:2 C + O<sub>2</sub> → 2 CO

Since CO is a gas, the reduction process can be driven by heating, exploiting the positive (favorable) [[Physics:Entropy|entropy]] of reaction. The [[Engineering:Ellingham diagram|Ellingham diagram]] shows that CO formation is favored over {{CO2}} in high temperatures.

==Use==

===Chemical industry===
Carbon monoxide is an [[Chemistry:Industrial gas|industrial gas]] that has many applications in bulk chemicals manufacturing.<ref>{{cite book|last1=Elschenbroich|first1=C.|title=Organometallics: A Concise Introduction|last2=Salzer|first2=A.|publisher=Wiley-VCH|year=2006|isbn=978-3-527-28165-7|edition=2nd|location=Weinheim}}</ref> Large quantities of aldehydes are produced by the [[Chemistry:Hydroformylation|hydroformylation]] reaction of [[Chemistry:Alkene|alkene]]s, carbon monoxide, and H<sub>2</sub>. Hydroformylation is coupled to the [[Chemistry:Shell higher olefin process|Shell higher olefin process]] to give precursors to detergents.

[[Chemistry:Phosgene|Phosgene]], useful for preparing isocyanates, polycarbonates, and polyurethanes, is produced by passing purified carbon monoxide and [[Chemistry:Chlorine|chlorine]] gas through a bed of porous [[Physics:Activated carbon|activated carbon]], which serves as a [[Chemistry:Catalyst|catalyst]].  World production of this compound was estimated to be 2.74 million tonnes in 1989.<ref name="Ullmann">{{Ullmann's|author1=Wolfgang Schneider|author2=Werner Diller|title=Phosgene|doi=10.1002/14356007.a19_411}}</ref>
:CO + Cl<sub>2</sub> → COCl<sub>2</sub>
[[Chemistry:Methanol|Methanol]] is produced by the [[Chemistry:Hydrogenation|hydrogenation]] of carbon monoxide. In a related reaction, the hydrogenation of carbon monoxide is coupled to C−C bond formation, as in the [[Chemistry:Fischer–Tropsch process|Fischer–Tropsch process]] where carbon monoxide is hydrogenated to liquid hydrocarbon fuels. This technology allows [[Earth:Coal|coal]] or biomass to be converted to diesel.

In the [[Chemistry:Cativa process|Cativa process]], carbon monoxide and methanol react in the presence of a homogeneous [[Chemistry:Iridium|iridium]] [[Chemistry:Catalyst|catalyst]] and [[Chemistry:Hydroiodic acid|hydroiodic acid]] to give [[Chemistry:Acetic acid|acetic acid]]. This process is responsible for most of the industrial production of acetic acid.

===Metallurgy===
{{Main|Chemistry:Metallurgy}} 

Carbon monoxide is a strong reductive agent and has been used in [[Engineering:Pyrometallurgy|pyrometallurgy]] to reduce [[Chemistry:Metal|metal]]s from [[Earth:Ore|ore]]s since ancient times. Carbon monoxide strips oxygen off metal oxides, reducing them to pure metal in high temperatures, forming [[Chemistry:Carbon dioxide|carbon dioxide]] in the process. Carbon monoxide is not usually supplied as is, in the gaseous phase, in the reactor, but rather it is formed in high temperature in presence of oxygen-carrying ore, or a carboniferous agent such as coke, and high temperature. The [[Engineering:Blast furnace|blast furnace]] process is a typical example of a process of reduction of metal from ore with carbon monoxide.

Likewise, [[Chemistry:Blast furnace gas|blast furnace gas]] collected at the top of blast furnace, still contains some 10% to 30% of carbon monoxide, and is used as fuel on Cowper stoves and on Siemens-Martin furnaces on [[Engineering:Open hearth furnace|open hearth steelmaking]].

===Lasers===
Carbon monoxide has also been used as a [[Physics:Active laser medium|lasing medium]] in high-powered infrared [[Physics:Laser|laser]]s.<ref>{{Cite journal | last1 = Ionin | first1 = A. | last2 = Kinyaevskiy | first2 = I. | last3 = Klimachev | first3 = Y. | last4 = Kotkov | first4 = A. | last5 = Kozlov | first5 = A. | s2cid = 112510554 | title = Novel mode-locked carbon monoxide laser system achieves high accuracy | doi = 10.1117/2.1201112.004016 | journal = SPIE Newsroom | year = 2012 }}</ref>

===Proposed use as fuel on Mars===
Carbon monoxide has been proposed for use as a fuel on Mars. Carbon monoxide/oxygen engines have been suggested for early surface transportation use as both carbon monoxide and oxygen can be straightforwardly produced from the carbon dioxide [[Astronomy:Atmosphere of Mars|atmosphere of Mars]] by zirconia [[Chemistry:Electrolysis|electrolysis]], without using any [[Astronomy:Water on Mars|Martian water resources]] to obtain hydrogen, which would be needed to make methane or any hydrogen-based fuel.<ref name="landis2001">{{cite journal|last=Landis|date=2001|title=Mars Rocket Vehicle Using In Situ Propellants|journal=Journal of Spacecraft and Rockets|volume=38|issue=5|pages=730–735|bibcode=2001JSpRo..38..730L|doi=10.2514/2.3739}}</ref>

==Biological and physiological properties==

===Physiology===
{{Main|Biology:Gasotransmitter}}
Carbon monoxide is a bioactive molecule which acts as a [[Biology:Gaseous signaling molecules|gaseous signaling molecule]]. It is naturally produced by many enzymatic and non-enzymatic pathways,<ref name=":2" /> the best understood of which is the catabolic action of [[Biology:Heme oxygenase|heme oxygenase]] on the [[Biology:Heme|heme]] derived from [[Biology:Hemoprotein|hemoprotein]]s such as [[Biology:Hemoglobin|hemoglobin]].<ref>{{Cite journal|last1=Ryter|first1=Stefan W.|last2=Alam|first2=Jawed|last3=Choi|first3=Augustine M. K.|date=2006-04-01|title=Heme Oxygenase-1/Carbon Monoxide: From Basic Science to Therapeutic Applications|url=https://journals.physiology.org/doi/full/10.1152/physrev.00011.2005|journal=Physiological Reviews|volume=86|issue=2|pages=583–650|doi=10.1152/physrev.00011.2005|pmid=16601269|issn=0031-9333}}</ref> Following the first report that carbon monoxide is a normal neurotransmitter in 1993,<ref name=":1" /> carbon monoxide has received significant clinical attention as a biological regulator.

Because of carbon monoxide's role in the body, abnormalities in its metabolism have been linked to a variety of diseases, including neurodegenerations, hypertension, heart failure, and pathological inflammation.<ref name="endogenous_co">{{Cite journal|last1=Wu|first1=L|last2=Wang|first2=R|date=December 2005|title=Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications|journal=Pharmacol Rev|volume=57|issue=4|pages=585–630|doi=10.1124/pr.57.4.3|pmid=16382109|s2cid=17538129}}</ref> In many tissues, carbon monoxide acts as anti-inflammatory, vasodilatory, and encouragers of neovascular growth.<ref>{{Cite journal|last1=Campbell|first1=Nicole K.|last2=Fitzgerald|first2=Hannah K.|last3=Dunne|first3=Aisling|date=2021-01-29|title=Regulation of inflammation by the antioxidant haem oxygenase 1|url=https://www.nature.com/articles/s41577-020-00491-x|journal=Nature Reviews Immunology|volume=21|issue=7|language=en|pages=411–425|doi=10.1038/s41577-020-00491-x|pmid=33514947|s2cid=231762031|issn=1474-1741}}</ref> In animal model studies, carbon monoxide reduced the severity of experimentally induced bacterial [[Medicine:Sepsis|sepsis]], pancreatitis, hepatic ischemia/reperfusion injury, colitis, osteoarthritis, lung injury, lung transplantation rejection, and neuropathic pain while promoting skin wound healing. Therefore, there is significant interest in the therapeutic potential of carbon monoxide becoming pharmaceutical agent and clinical standard of care.<ref name=":3">{{Cite journal|last1=Motterlini|first1=Roberto|last2=Otterbein|first2=Leo E.|date=2010|title=The therapeutic potential of carbon monoxide|url=https://www.nature.com/articles/nrd3228|journal=Nature Reviews Drug Discovery|language=en|volume=9|issue=9|pages=728–743|doi=10.1038/nrd3228|pmid=20811383|s2cid=205477130|issn=1474-1784}}</ref>

====Medicine====
{{Main|Chemistry:Carbon monoxide-releasing molecules}}
Studies involving carbon monoxide have been conducted in many laboratories throughout the world for its anti-inflammatory and cytoprotective properties.<ref>{{Cite journal|last1=Motterlini|first1=Roberto|last2=Foresti|first2=Roberta|date=2017-01-11|title=Biological signaling by carbon monoxide and carbon monoxide-releasing molecules|journal=American Journal of Physiology. Cell Physiology|volume=312|issue=3|pages=C302–C313|doi=10.1152/ajpcell.00360.2016|pmid=28077358|issn=0363-6143|doi-access=free}}</ref> These properties have the potential to be used to prevent the development of a series of pathological conditions including ischemia reperfusion injury, transplant rejection, atherosclerosis, severe sepsis, severe malaria, or autoimmunity.<ref name=":3" /> Many pharmaceutical drug delivery initiatives have developed methods to safely administer carbon monoxide, and subsequent controlled clinical trials have evaluated the therapeutic effect of carbon monoxide.<ref>{{Cite journal|last1=Hopper|first1=Christopher P.|last2=Meinel|first2=Lorenz|last3=Steiger|first3=Christoph|last4=Otterbein|first4=Leo E.|date=2018-10-11|title=Where is the Clinical Breakthrough of Heme Oxygenase-1 / Carbon Monoxide Therapeutics?|url=https://www.eurekaselect.com/164002/article|journal=Current Pharmaceutical Design|language=en|volume=24|issue=20|pages=2264–2282|doi=10.2174/1381612824666180723161811|pmid=30039755|s2cid=51712930}}</ref>

===Microbiology===
Microbiota may also utilize carbon monoxide as a [[Biology:Gasotransmitter|gasotransmitter]].<ref>{{Cite journal|last1=Wareham|first1=Lauren K.|last2=Southam|first2=Hannah M.|last3=Poole|first3=Robert K.|date=2018-09-06|title=Do nitric oxide, carbon monoxide and hydrogen sulfide really qualify as 'gasotransmitters' in bacteria?|url= |journal=Biochemical Society Transactions|volume=46|issue=5|pages=1107–1118|doi=10.1042/BST20170311|pmid=30190328|pmc=6195638|issn=0300-5127}}</ref> Carbon monoxide sensing is a signaling pathway facilitated by proteins such as [[Biology:CooA|CooA]].<ref>{{Cite journal|author1=Roberts, G. P. |author2=Youn, H. |author3=Kerby, R. L. |title=CO-Sensing Mechanisms|journal=Microbiology and Molecular Biology Reviews|year=2004|volume=68|pages=453–473|doi=10.1128/MMBR.68.3.453-473.2004|pmid=15353565|issue=3|pmc=515253}}</ref><ref>{{Cite journal|last1=Shimizu|first1=Toru|last2=Lengalova|first2=Alzbeta|last3=Martínek|first3=Václav|last4=Martínková|first4=Markéta|date=2019-12-09|title=Heme: emergent roles of heme in signal transduction, functional regulation and as catalytic centres|url=https://pubs.rsc.org/en/content/articlelanding/2019/cs/c9cs00268e|journal=Chemical Society Reviews|language=en|volume=48|issue=24|pages=5624–5657|doi=10.1039/C9CS00268E|pmid=31748766|s2cid=208217502|issn=1460-4744}}</ref><ref>{{Cite journal|last1=Shimizu|first1=Toru|last2=Huang|first2=Dongyang|last3=Yan|first3=Fang|last4=Stranava|first4=Martin|last5=Bartosova|first5=Martina|last6=Fojtíková|first6=Veronika|last7=Martínková|first7=Markéta|date=2015-07-08|title=Gaseous O2, NO, and CO in Signal Transduction: Structure and Function Relationships of Heme-Based Gas Sensors and Heme-Redox Sensors|url=https://doi.org/10.1021/acs.chemrev.5b00018|journal=Chemical Reviews|volume=115|issue=13|pages=6491–6533|doi=10.1021/acs.chemrev.5b00018|pmid=26021768|issn=0009-2665}}</ref> The scope of the biological roles for carbon monoxide sensing is still unknown.

The human microbiome produces, consumes, and responds to carbon monoxide.<ref name=":2">{{Cite journal|last1=Hopper|first1=Christopher P.|last2=De La Cruz|first2=Ladie Kimberly|last3=Lyles|first3=Kristin V.|last4=Wareham|first4=Lauren K.|last5=Gilbert|first5=Jack A.|last6=Eichenbaum|first6=Zehava|last7=Magierowski|first7=Marcin|last8=Poole|first8=Robert K.|last9=Wollborn|first9=Jakob|last10=Wang|first10=Binghe|date=2020-12-23|title=Role of Carbon Monoxide in Host–Gut Microbiome Communication|url=https://doi.org/10.1021/acs.chemrev.0c00586|journal=Chemical Reviews|volume=120|issue=24|pages=13273–13311|doi=10.1021/acs.chemrev.0c00586|pmid=33089988|s2cid=224824871|issn=0009-2665}}</ref> For example, in certain bacteria, carbon monoxide is produced via the [[Chemistry:Redox|reduction]] of carbon dioxide by the enzyme [[Biology:Carbon monoxide dehydrogenase|carbon monoxide dehydrogenase]] with favorable [[Biology:Bioenergetics|bioenergetics]] to power downstream cellular operations.<ref>{{Cite book|title=Bioorganometallics: Biomolecules, Labeling, Medicine|editor=Jaouen, G.|publisher=Wiley-VCH|location=Weinheim|year=2006|isbn=978-3-527-30990-0}}</ref><ref name=":2" /> In another example, carbon monoxide is a nutrient for [[Biology:Methanogen|methanogen]]ic archaea which reduce it to methane using hydrogen.<ref>{{Cite journal|author=Thauer, R. K. |title=Biochemistry of methanogenesis: a tribute to Marjory Stephenson. 1998 Marjory Stephenson Prize Lecture|year=1998|journal=Microbiology|volume=144|issue=9|pages=2377–2406|url=http://mic.microbiologyresearch.org/deliver/fulltext/micro/144/9/mic-144-9-2377.pdf?itemId=/content/journal/micro/10.1099/00221287-144-9-2377|format=Free|doi=10.1099/00221287-144-9-2377|pmid=9782487|doi-access=free}}</ref>

Carbon monoxide has certain antimicrobial properties which have been studied to treat against infectious diseases.<ref name=":2" />

====Food science====
Carbon monoxide is used in [[Chemistry:Modified atmosphere|modified atmosphere]] packaging systems in the US, mainly with fresh meat products such as beef, pork, and fish to keep them looking fresh. The benefit is two-fold, carbon monoxide protects against microbial spoilage and it enhances the meat color for consumer appeal.<ref name=":4">{{Cite journal|last1=Van Rooyen|first1=Lauren Anne|last2=Allen|first2=Paul|last3=O'Connor|first3=David I.|date=October 2017|title=The application of carbon monoxide in meat packaging needs to be re-evaluated within the EU: An overview|journal=Meat Science|language=en|volume=132|pages=179–188|doi=10.1016/j.meatsci.2017.03.016|pmid=28465017|doi-access=free}}</ref> The carbon monoxide combines with [[Biology:Myoglobin|myoglobin]] to form carboxymyoglobin, a bright-cherry-red pigment. Carboxymyoglobin is more stable than the oxygenated form of myoglobin, oxymyoglobin, which can become oxidized to the brown pigment [[Biology:Metmyoglobin|metmyoglobin]]. This stable red color can persist much longer than in normally packaged meat. Typical levels of carbon monoxide used in the facilities that use this process are between 0.4% and 0.5%.<ref name=":4" />

The technology was first given "[[Chemistry:Generally recognized as safe|generally recognized as safe]]" (GRAS) status by the U.S. Food and Drug Administration (FDA) in 2002 for use as a secondary packaging system, and does not require labeling. In 2004, the FDA approved CO as primary packaging method, declaring that CO does not mask spoilage odor.<ref name="Meatsci2005_eilert">{{Cite journal|author=Eilert EJ|year=2005|title=New packaging technologies for the 21st century|journal=Journal of Meat Science|volume=71|issue=1|pages=122–127|doi=10.1016/j.meatsci.2005.04.003|pmid=22064057}}</ref> The process is currently unauthorized in many other countries, including Japan, [[Place:Singapore|Singapore]], and the [[Social:European Union|European Union]].<ref>{{Cite news|date=November 14, 2007|title=Proof in the Pink? Meat Treated to Give It Fresh Look|agency=ABC News|url=https://abcnews.go.com/GMA/Consumer/Story?id=3863064&page=1|access-date=May 5, 2009}}</ref><ref>{{Cite book|url=http://www.meatami.com/ht/a/GetDocumentAction/i/40141|title=Carbon Monoxide in Meat Packaging: Myths and Facts|publisher=American Meat Institute|year=2008|access-date=May 5, 2009|archive-url=https://web.archive.org/web/20110714064101/http://www.meatami.com/ht/a/GetDocumentAction/i/40141|archive-date=2011-07-14|url-status=dead}}</ref><ref>{{Cite web|title=CO in packaged meat|url=http://www.carbonmonoxidekills.com/packages_meat.htm|url-status=dead|archive-url=https://web.archive.org/web/20100926055707/http://www.carbonmonoxidekills.com/packages_meat.htm|archive-date=September 26, 2010|access-date=May 5, 2009|publisher=Carbon Monoxide Kills Campaign}}</ref>

===Toxicity===
<!--Please keep this section short, there is no need to elaborate here. Details will be more useful in the [[Medicine:Carbon monoxide poisoning|Carbon monoxide poisoning]] article.-->
{{Main|Medicine:Carbon monoxide poisoning}}
{{see also|Carboxyhemoglobin}}[[Medicine:Carbon monoxide poisoning|Carbon monoxide poisoning]] is the most common type of fatal air poisoning in many countries.<ref name="Toxicology2002-omaye">{{Cite journal|author=Omaye ST|year=2002|title=Metabolic modulation of carbon monoxide toxicity|journal=Toxicology|volume=180|issue=2|pages=139–150|doi=10.1016/S0300-483X(02)00387-6|pmid=12324190}}</ref> The Centers for Disease Control and Prevention estimates that several thousand people go to hospital emergency rooms every year to be treated for carbon monoxide poisoning.<ref>Centers for Disease Control and Prevention, National Environmental Public Health Tracking Network, [http://ephtracking.cdc.gov/showCarbonMonoxideLanding.action Carbon Monoxide Poisoning] {{Webarchive|url=https://web.archive.org/web/20091210052437/http://ephtracking.cdc.gov/showCarbonMonoxideLanding.action |date=2009-12-10 }}, accessed 2009-12-04</ref> According to the Florida Department of Health, "every year more than 500 Americans die from accidental exposure to carbon monoxide and thousands more across the U.S. require emergency medical care for non-fatal carbon monoxide poisoning."<ref name="EPHT">{{cite web|title=Tracking Carbon Monoxide|url=http://www.doh.state.fl.us/Environment/programs/Environmental_Public_Health_Tracking/HTML_pages/Data_Carbon_Monoxide.html|url-status=dead|archive-url=https://web.archive.org/web/20110927164735/http://www.doh.state.fl.us/Environment/programs/Environmental_Public_Health_Tracking/HTML_pages/Data_Carbon_Monoxide.html|archive-date=2011-09-27|publisher=Environmental Public Health Tracking – Florida Dept. of Health}}</ref> The American Association of Poison Control Centers (AAPCC) reported 15,769 cases of carbon monoxide poisoning resulting in 39 deaths in 2007.<ref>[https://web.archive.org/web/20101204014524/http://www.aapcc.org/dnn/NPDSPoisonData/AnnualReports/tabid/125/Default.aspx "AAPCC Annual Data Reports 2007"]. American Association of Poison Control Centers.</ref> In 2005, the CPSC reported 94 generator-related carbon monoxide poisoning deaths.<ref name="cpsc" />

Carbon monoxide is colorless, odorless, and tasteless. As such, it is relatively undetectable. It readily combines with [[Biology:Hemoglobin|hemoglobin]] to produce [[Chemistry:Carboxyhemoglobin|carboxyhemoglobin]] which potentially affects [[Physics:Gas exchange|gas exchange]]; therefore exposure can be highly toxic. Concentrations as low as 667&nbsp;ppm may cause up to 50% of the body's hemoglobin to convert to carboxyhemoglobin.<ref>{{Cite journal|last1=Tikuisis|first1=P|last2=Kane|first2=DM|last3=McLellan|first3=TM|last4=Buick|first4=F|last5=Fairburn|first5=SM|year=1992|title=Rate of formation of carboxyhemoglobin in exercising humans exposed to carbon monoxide|journal=Journal of Applied Physiology|volume=72|issue=4|pages=1311–9|doi=10.1152/jappl.1992.72.4.1311|pmid=1592720}}</ref> A level of 50% carboxyhemoglobin may result in seizure, coma, and fatality.<ref name="Blumenthal">{{Cite journal|last=Blumenthal|first=Ivan|date=1 June 2001|title=Carbon monoxide poisoning|journal=J R Soc Med|volume=94|issue=6|pages=270–272|doi=10.1177/014107680109400604|pmc=1281520|pmid=11387414}}</ref> In the United States, the [[Organization:Occupational Safety and Health Administration|OSHA]] limits long-term workplace exposure levels above 50 ppm.<ref>{{Cite web|title=OSHA CO guidelines|url=https://www.osha.gov/SLTC/healthguidelines/carbonmonoxide/recognition.html|url-status=dead|archive-url=https://web.archive.org/web/20100126010501/https://www.osha.gov/SLTC/healthguidelines/carbonmonoxide/recognition.html|archive-date=January 26, 2010|access-date=May 27, 2009|publisher=OSHA}}</ref>

In addition to affecting oxygen delivery, carbon monoxide also binds to other [[Biology:Hemoprotein|hemoprotein]]s such as [[Biology:Myoglobin|myoglobin]] and [[Biology:Mitochondrion|mitochondrial]] cytochrome oxidase, metallic and non-metallic cellular targets to affect many cell operations.

==== Weaponization ====
In ancient history, Hannibal executed Roman prisoners with coal fumes during the Second Punic War.<ref name=":1" />

Carbon monoxide had been used for genocide during [[Social:The Holocaust|the Holocaust]] at some extermination camps, the most notable by gas vans in Chełmno, and in the Action T4 "euthanasia" program.<ref>{{Cite book|author=Kitchen, Martin|url=https://books.google.com/books?id=_04R6DoPxLoC&pg=PT323|title=A history of modern Germany, 1800–2000|publisher=Wiley-Blackwell|year=2006|isbn=978-1-4051-0041-0|page=323}}</ref>

==See also==
*{{annotated link|Chemistry:Carbon monoxide (data page)}}
*{{annotated link|Chemistry:Breath carbon monoxide}}
*{{annotated link|Chemistry:Carbon monoxide detector}}
*[[Chemistry:Hydrocarbonate (gas)|Hydrocarbonate (gas)]]
*{{annotated link|Earth:Criteria air pollutants}}
*{{annotated link|Chemistry:List of highly toxic gases}}
*Smoker's paradox
*{{annotated link|Organization:Undersea and Hyperbaric Medical Society}} – hyperbaric treatment for CO poisoning
*{{annotated link|Organization:Rubicon Foundation}} research articles on CO poisoning

==References==
{{Reflist|35em}}

==External links==
* [https://earth.nullschool.net/#current/chem/surface/level/overlay=cosc/winkel3 Global map of carbon monoxide distribution]
* [https://web.archive.org/web/20110720030754/http://courses.chem.psu.edu/chem210/mol-gallery/co/co.html Explanation of the structure]
* [https://inchem.org/documents/icsc/icsc/eics0023.htm International Chemical Safety Card 0023]
* [https://www.cdc.gov/niosh/npg/npgd0105.html CDC NIOSH Pocket Guide to Chemical Hazards: Carbon monoxide]—[[Organization:National Institute for Occupational Safety and Health|National Institute for Occupational Safety and Health]] (NIOSH), US Centers for Disease Control and Prevention (CDC)
** [https://www.cdc.gov/niosh/topics/co-comp/ Carbon Monoxide—NIOSH Workplace Safety and Health Topic]—CDC
** [https://www.cdc.gov/co/faqs.htm Carbon Monoxide Poisoning—Frequently Asked Questions]—CDC
* [https://web.archive.org/web/20060516154404/http://physchem.ox.ac.uk/MSDS/CA/carbon_monoxide.html External MSDS data sheet]
* [https://web.archive.org/web/20081218075252/http://www.homesafe.com/coalert/detect.htm Carbon Monoxide Detector Placement]
* [http://mattson.creighton.edu/CO/index.html Microscale Gas Chemistry Experiments with Carbon Monoxide]
* {{Cite journal |title=Instant insight: Don't blame the messenger |url=http://www.rsc.org/Publishing/Journals/cb/Volume/2007/11/Dont_blame_the_messenger.asp |archive-url=https://web.archive.org/web/20071028055303/http://www.rsc.org/Publishing/Journals/cb/Volume/2007/11/Dont_blame_the_messenger.asp |archive-date=28 October 2007 |url-status=dead |date=18 October 2007 |journal=Chemical Biology |issue= 11: Research News |access-date=27 October 2019 |quote=Outlining the physiology of carbon monoxide from the [[Organization:Royal Society of Chemistry|Royal Society of Chemistry]].}}

{{Oxides}}
{{Oxides of carbon}}
{{Neurotransmitters}}
{{Molecules detected in outer space}}
{{Carbon compounds}}
{{oxygen compounds}}

{{DEFAULTSORT:Carbon Monoxide}}
[[Category:Carbon monoxide| ]]
[[Category:Gaseous signaling molecules]]
[[Category:Industrial gases]]
[[Category:Oxocarbons]]
[[Category:Smog]]
[[Category:Toxicology]]
[[Category:Diatomic molecules]]

{{Sourceattribution|Carbon monoxide}}