Editing Carbon (section)

==Compounds==
{{Main|Carbon compounds}}

===Organic compounds===
[[File:Methane-2D-stereo.svg|thumb|left|upright=0.7|Structural formula of [[methane]], the simplest possible organic compound.]]
[[File:Auto-and heterotrophs.png|thumb|upright=1.35|Correlation between the ''carbon cycle'' and formation of organic compounds. In plants, carbon dioxide formed by carbon fixation can join with water in [[photosynthesis]] (<span style="color:green;">green</span>) to form organic compounds, which can be used and further converted by both plants and animals.]]
Carbon can form very long chains of interconnecting [[carbon–carbon bond]]s, a property that is called [[catenation]]. Carbon-carbon bonds are strong and stable. Through [p[catenation, carbon forms a countless number of compounds. A tally of unique compounds shows that more contain carbon than do not.<ref name="Burrows-2017">{{cite book |last1=Burrows |first1=A. |last2=Holman |first2=J. |last3=Parsons |first3=A. |last4=Pilling |first4=G. |last5=Price |first5=G. |title=Chemistry3: Introducing Inorganic, Organic and Physical Chemistry |publisher=Oxford University Press |year=2017 |isbn=978-0-19-873380-5 |page=70 |url=https://books.google.com/books?id=YzbjDQAAQBAJ&pg=PA70 |access-date=2017-05-07 |url-status=live |archive-url=https://web.archive.org/web/20171122173131/https://books.google.com/books?id=YzbjDQAAQBAJ&pg=PA70 |archive-date=2017-11-22}}</ref>

The simplest form of an organic molecule is the hydrocarbon—a large family of organic molecules that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms, known as [[heteroatom]]s. Common heteroatoms that appear in organic compounds include oxygen, nitrogen, sulfur, phosphorus, and the nonradioactive halogens, as well as the metals lithium and magnesium. Organic compounds containing bonds to metal are known as organometallic compounds (''see below''). Certain groupings of atoms, often including heteroatoms, recur in large numbers of organic compounds. These collections, known as ''[[functional group]]s'', confer common reactivity patterns and allow for the systematic study and categorization of organic compounds. Chain length, shape and functional groups all affect the properties of organic molecules.<ref>Mannion pp. 27–51</ref>

In most stable compounds of carbon (and nearly all stable ''organic'' compounds), carbon obeys the [[octet rule]] and is ''tetravalent'', meaning that a carbon atom forms a total of four covalent bonds (which may include double and triple bonds). Exceptions include a small number of stabilized ''carbocations'' (three bonds, positive charge), ''radicals'' (three bonds, neutral), ''carbanions'' (three bonds, negative charge) and ''carbenes'' (two bonds, neutral), although these species are much more likely to be encountered as unstable, reactive intermediates.<ref name="Claydentext">{{Clayden}}</ref>

Carbon occurs in all known organic life and is the basis of [[organic chemistry]]. When united with hydrogen, it forms various hydrocarbons that are important to industry as refrigerants, lubricants, solvents, as chemical feedstock for the manufacture of plastics and petrochemicals, and as fossil fuels.<ref name="Claydentext" />

When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, [[lignan]]s, [[chitin]]s, alcohols, fats, aromatic [[ester]]s, [[carotenoid]]s and [[terpene]]s. With nitrogen, it forms [[alkaloid]]s, and with the addition of sulfur also it forms antibiotics, [[amino acid]]s, and rubber products. With the addition of phosphorus to these other elements, it forms [[DNA]] and [[RNA]], the chemical-code carriers of life, and [[adenosine triphosphate]] (ATP), the most important energy-transfer molecule in all living cells.<ref>Mannion pp. 84–91</ref>  [[Norman Horowitz]], head of the [[Viking program|Mariner and Viking missions to Mars]] (1965–1976), considered that the unique characteristics of carbon made it unlikely that any other element could replace carbon, even on another planet, to generate the biochemistry necessary for life.<ref>Norman H. Horowitz (1986) To Utopia and Back; the search for life in the solar system (Astronomy Series) W. H. Freeman & Co (Sd), NY, {{ISBN|978-0-7167-1766-9}}</ref>

===Inorganic compounds===
Commonly carbon-containing compounds which are associated with minerals or which do not contain bonds to the other carbon atoms, halogens, or hydrogen, are treated separately from classical organic compounds; the definition is not rigid, and the classification of some compounds can vary from author to author (see reference articles above). Among these are the simple oxides of carbon. The most prominent oxide is carbon dioxide ({{CO2}}). This was once the principal constituent of the [[paleoatmosphere]], but is a minor component of the [[Earth's atmosphere]] today.<ref>{{cite journal |date=1982 |title=The prebiological paleoatmosphere: stability and composition |journal=Origins of Life and Evolution of Biospheres |volume=12 |pages=245–259 |doi=10.1007/BF00926894 |pmid=7162799 |issue=3 |bibcode=1982OrLi...12..245L |s2cid=20097153 |last1=Levine |first1=Joel S. |last2=Augustsson |first2=Tommy R. |last3=Natarajan |first3=Murali}}</ref> Dissolved in water, it forms [[carbonic acid]] ({{chem|H|2|C|O|3}}), but as most compounds with multiple single-bonded oxygens on a single carbon it is unstable.<ref>{{cite journal |author=Loerting, T. |author1-link=Thomas Loerting |date=2001 |title=On the Surprising Kinetic Stability of Carbonic Acid |journal=Angew. Chem. Int. Ed. |volume=39 |pages=891–895 |doi=10.1002/(SICI)1521-3773(20000303)39:5<891::AID-ANIE891>3.0.CO;2-E |pmid=10760883 |issue=5 |display-authors=1 |last2=Tautermann |first2=Christofer |last3=Kroemer |first3=Romano T. |last4=Kohl |first4=Ingrid |last5=Hallbrucker |first5=Andreas |last6=Mayer |first6=Erwin |last7=Liedl |first7=Klaus R.}}</ref> Through this intermediate, though, resonance-stabilized carbonate [[ion]]s are produced. Some important minerals are carbonates, notably [[calcite]]. [[Carbon disulfide]] ({{chem|C|S|2}}) is similar.{{sfn|Greenwood|Earnshaw|1997|pages=289-292}}

The other common oxide is carbon monoxide (CO). It is formed by incomplete combustion, and is a colorless, odorless gas. The molecules each contain a triple bond and are fairly [[polar molecule|polar]], resulting in a tendency to bind permanently to hemoglobin molecules, displacing oxygen, which has a lower binding affinity.<ref>{{cite journal |author=Haldane J. |date=1895 |title=The action of carbonic oxide on man |journal=Journal of Physiology |volume=18 |pages=430–462 |pmid=16992272 |issue=5–6 |pmc=1514663 |doi=10.1113/jphysiol.1895.sp000578}}</ref><ref>{{cite journal |date=2003 |title=The clinical toxicology of carbon monoxide |journal=Toxicology |issue=1 |pages=25–38 |volume=187 |pmid=12679050 |doi=10.1016/S0300-483X(03)00005-2 |last1=Gorman |first1=D. |last2=Drewry |first2=A. |last3=Huang |first3=Y. L. |last4=Sames |first4=C.|bibcode=2003Toxgy.187...25G }}</ref> [[Cyanide]] (CN{{sup|−}}), has a similar structure, but behaves much like a [[halide]] ion ([[pseudohalogen]]). For example, it can form the nitride [[cyanogen]] molecule ((CN){{sub|2}}), similar to diatomic halides. Likewise, the heavier analog of cyanide, [[cyaphide]] (CP{{sup|−}}), is also considered inorganic, though most simple derivatives are highly unstable. Other uncommon oxides are [[carbon suboxide]] ({{chem|C|3|O|2}}),<ref>{{cite web |title=Compounds of carbon: carbon suboxide |url=http://www.webelements.com/webelements/compounds/text/C/C3O2-504643.html |access-date=2007-12-03 |url-status=live |archive-url=https://web.archive.org/web/20071207230312/http://www.webelements.com/webelements/compounds/text/C/C3O2-504643.html |archive-date=2007-12-07}}</ref> the unstable [[dicarbon monoxide]] (C{{sub|2}}O),<ref>{{cite journal |last1=Bayes |first1=K. |title=Photolysis of Carbon Suboxide |journal=[[Journal of the American Chemical Society]] |volume=83 |date=1961 |pages=3712–3713 |doi=10.1021/ja01478a033 |issue=17|bibcode=1961JAChS..83.3712B }}</ref><ref>{{cite journal |author=Anderson D. J. |title=Photodissociation of Carbon Suboxide |journal=[[Journal of Chemical Physics]] |volume=94 |date=1991 |pages=7852–7867 |issue=12 |bibcode=1991JChPh..94.7857A |doi=10.1063/1.460121 |last2=Rosenfeld |first2=R. N.}}</ref> [[carbon trioxide]] (CO{{sub|3}}),<ref>{{cite journal |title=A theoretical study of the structure and properties of carbon trioxide |last1=Sabin |first1=J. R. |journal=[[Chemical Physics Letters]] |volume=11 |issue=5 |pages=593–597 |date=1971 |doi=10.1016/0009-2614(71)87010-0 |bibcode=1971CPL....11..593S |last2=Kim |first2=H.}}</ref><ref>{{cite journal |title=Carbon Trioxide: Its Production, Infrared Spectrum, and Structure Studied in a Matrix of Solid CO{{sub |2}} |journal=Journal of Chemical Physics|date=1966|volume=45|issue=12|pages=4469–4481|doi=10.1063/1.1727526|bibcode=1966JChPh..45.4469M |author=Moll N. G.|author2=Clutter D. R.|author3=Thompson W. E.}}</ref> [[cyclopentanepentone]] (C{{sub|5}}O{{sub|5}}),<ref name="Fatiadi-1963">{{cite journal |title=Cyclic Polyhydroxy Ketones. I. Oxidation Products of Hexahydroxybenzene (Benzenehexol) |journal=Journal of Research of the National Bureau of Standards Section A |volume=67A |issue=2 |date=1963 |pages=153–162 |url=http://nvl.nist.gov/pub/nistpubs/jres/067/2/V67.N02.A06.pdf |url-status=dead |access-date=2009-03-21 |doi=10.6028/jres.067A.015 |pmid=31580622 |pmc=6640573 |first1=Alexander J. |last1=Fatiadi |last2=Isbell |first2=Horace S. |last3=Sager |first3=William F. |archive-url=https://web.archive.org/web/20090325204012/http://nvl.nist.gov/pub/nistpubs/jres/067/2/V67.N02.A06.pdf |archive-date=2009-03-25}}</ref> [[cyclohexanehexone]] (C{{sub|6}}O{{sub|6}}),<ref name="Fatiadi-1963"/> and [[mellitic anhydride]] (C{{sub|12}}O{{sub|9}}). However, mellitic anhydride is the triple acyl anhydride of mellitic acid; moreover, it contains a benzene ring. Thus, many chemists consider it to be organic.

With reactive metals, such as [[tungsten]], carbon forms either [[carbide]]s (C{{sup|4−}}) or [[acetylide]]s ({{chem|C|2|2-}}) to form alloys with high melting points. These anions are also associated with methane and [[acetylene]], both very weak acids. With an electronegativity of 2.5,<ref>{{cite book |first=L. |last=Pauling |title=The Nature of the Chemical Bond |url=https://archive.org/details/natureofchemical00paul |url-access=registration |edition=3rd |publisher=Cornell University Press |location=Ithaca, NY |date=1960 |page=[https://archive.org/details/natureofchemical00paul/page/93 93] |isbn=978-0-8014-0333-0}}</ref> carbon prefers to form [[covalent bond]]s. A few carbides are covalent lattices, like [[carborundum]] (SiC), which resembles diamond. Nevertheless, even the most polar and salt-like of carbides are not completely ionic compounds.{{sfn|Greenwood|Earnshaw|1997|pages=297-301}}

===Organometallic compounds===
{{Main|Organometallic chemistry}}
Organometallic compounds by definition contain at least one carbon-metal covalent bond. A wide range of such compounds exist; major classes include simple alkyl-metal compounds (for example, [[tetraethyllead]]), η{{sup|2}}-alkene compounds (for example, [[Zeise's salt]]), and η{{sup|3}}-allyl compounds (for example, [[allylpalladium chloride dimer]]); [[metallocene]]s containing cyclopentadienyl ligands (for example, [[ferrocene]]); and [[transition metal carbene complex]]es. Many [[metal carbonyl]]s and [[cyanometalate|metal cyanides]] exist (for example, [[tetracarbonylnickel]] and [[potassium ferricyanide]]); some workers consider metal carbonyl and cyanide complexes without other carbon ligands to be purely inorganic, and not organometallic. However, most organometallic chemists consider metal complexes with any carbon ligand, even 'inorganic carbon' (e.g., carbonyls, cyanides, and certain types of carbides and acetylides) to be organometallic in nature. Metal complexes containing organic ligands without a carbon-metal covalent bond (e.g., metal carboxylates) are termed ''metalorganic'' compounds.

While carbon is understood to strongly prefer formation of four covalent bonds, other exotic bonding schemes are also known. [[Carborane]]s are highly stable dodecahedral derivatives of the [B<sub>12</sub>H<sub>12</sub>]<sup>2-</sup> unit, with one BH replaced with a CH<sup>+</sup>. Thus, the carbon is bonded to five boron atoms and one hydrogen atom. The cation [(Ph{{sub|3}}PAu){{sub|6}}C]{{sup|2+}} contains an octahedral carbon bound to six phosphine-gold fragments. This phenomenon has been attributed to the [[aurophilicity]] of the gold ligands, which provide additional stabilization of an otherwise labile species.<ref>{{cite journal |author=Scherbaum, Franz |journal=[[Angew. Chem. Int. Ed. Engl.]] |volume=27 |issue=11 |pages=1544–1546 |date=1988 |doi=10.1002/anie.198815441 |title="Aurophilicity" as a consequence of Relativistic Effects: The Hexakis(triphenylphosphaneaurio)methane Dication [(Ph{{sub |3}}PAu){{sub|6}}C]{{sup|2+}} |display-authors=1|last2=Grohmann|first2=Andreas|last3=Huber|first3=Brigitte|last4=Krüger|first4=Carl|last5=Schmidbaur|first5=Hubert}}</ref> In nature, the iron-molybdenum cofactor ([[FeMoco]]) responsible for microbial [[nitrogen fixation]] likewise has an octahedral carbon center (formally a carbide, C(-IV)) bonded to six iron atoms. In 2016, it was confirmed that, in line with earlier theoretical predictions, the [[hexamethylbenzene|hexamethylbenzene dication]] contains a carbon atom with six bonds. More specifically, the dication could be described structurally by the formulation [MeC(η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)]<sup>2+</sup>, making it an "organic [[metallocene]]" in which a MeC<sup>3+</sup> fragment is bonded to a η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub><sup>−</sup> fragment through all five of the carbons of the ring.<ref>{{cite magazine |url=http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html?type=paidArticleContent |url-status=live |title=Six bonds to carbon: Confirmed |first=Stephen K. |last=Ritter |magazine=Chemical & Engineering News |archive-url=https://web.archive.org/web/20170109183800/http://cen.acs.org/articles/94/i49/Six-bonds-carbon-Confirmed.html?type=paidArticleContent |archive-date=2017-01-09}}</ref>

[[File:Akiba's "hypervalent carbon" compound.png|thumb|This anthracene derivative contains a carbon atom with 5 formal electron pairs around it.]]
It is important to note that in the cases above, each of the bonds to carbon contain less than two formal electron pairs. Thus, the formal electron count of these species does not exceed an octet. This makes them hypercoordinate but not hypervalent. Even in cases of alleged 10-C-5 species (that is, a carbon with five ligands and a formal electron count of ten), as reported by Akiba and co-workers,<ref>{{cite journal |last1=Yamashita |first1=Makoto |last2=Yamamoto |first2=Yohsuke |last3=Akiba |first3=Kin-ya |last4=Hashizume |first4=Daisuke |last5=Iwasaki |first5=Fujiko |last6=Takagi |first6=Nozomi |last7=Nagase |first7=Shigeru |date=2005-03-01 |title=Syntheses and Structures of Hypervalent Pentacoordinate Carbon and Boron Compounds Bearing an Anthracene Skeleton − Elucidation of Hypervalent Interaction Based on X-ray Analysis and DFT Calculation |journal=Journal of the American Chemical Society |volume=127 |issue=12 |pages=4354–4371 |doi=10.1021/ja0438011 |pmid=15783218 |bibcode=2005JAChS.127.4354Y |issn=0002-7863}}</ref> electronic structure calculations conclude that the electron population around carbon is still less than eight, as is true for other compounds featuring four-electron [[three-center bond]]ing.