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In chemistry, an oxocarbon or oxide of carbon is a
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one elemen ...
consisting only of carbon and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well ...
. The simplest and most common oxocarbons are
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
(CO) and carbon dioxide (). Many other stable (practically if not thermodynamically) or
metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...
oxides of carbon are known, but they are rarely encountered, such as
carbon suboxide Carbon suboxide, or tricarbon dioxide, is an organic, oxygen-containing chemical compound with formula and structure . Its four cumulative double bonds make it a cumulene. It is one of the stable members of the series of linear oxocarbons , whi ...
( or ) and
mellitic anhydride Mellitic anhydride, the anhydride of mellitic acid, is an organic compound with the formula C12O9. Containing no other elements (e.g., hydrogen) besides carbon and oxygen, mellitic anhydride is an oxide of carbon ( oxocarbon), and, along with C ...
(). Many other oxides are known today, most of them synthesized since the 1960s. Some of these new oxides are stable at room temperature. Some are
metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...
or stable only at very low temperatures, but decompose to simpler oxocarbons when warmed. Many are inherently unstable and can be observed only momentarily as intermediates in chemical reactions or are so reactive that they exist only in gas phase or have only been detected by
matrix isolation Matrix isolation is an experimental technique used in chemistry and physics. It generally involves a material being trapped within an unreactive matrix. A ''host'' matrix is a continuous solid phase in which ''guest'' particles (atoms, molecules, i ...
.
Graphene oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...
and other stable
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...
ic carbon oxides with unbounded molecular structures exist.


Overview

Carbon dioxide (CO2) occurs widely in nature, and was incidentally produced by humans since pre-historical times, by breathing, the
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
of carbon-containing substances and
fermentation Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food ...
of foods such as beer and
bread Bread is a staple food prepared from a dough of flour (usually wheat) and water, usually by baking. Throughout recorded history and around the world, it has been an important part of many cultures' diet. It is one of the oldest human-made food ...
. It was gradually recognized as a chemical substance, formerly called ''spiritus sylvestris'' ("forest spirit") or "fixed air", by various chemists in the 17th and 18th centuries. Carbon monoxide may occur in combustion, too, and was used (though not recognized) since antiquity for the
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a c ...
of
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fr ...
from its
ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
s. Like the dioxide, it was described and studied in the West by various
alchemist Alchemy (from Arabic: ''al-kīmiyā''; from Ancient Greek: χυμεία, ''khumeía'') is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practiced in China, India, the Muslim world ...
s and chemists since the Middle Ages. Its true composition was discovered by William Cruikshank in 1800. Carbon suboxide was discovered by Benjamin Brodie in 1873, by passing electric current through carbon dioxide. The fourth "classical" oxide,
mellitic anhydride Mellitic anhydride, the anhydride of mellitic acid, is an organic compound with the formula C12O9. Containing no other elements (e.g., hydrogen) besides carbon and oxygen, mellitic anhydride is an oxide of carbon ( oxocarbon), and, along with C ...
(C12O9), was apparently obtained by
Liebig Justus Freiherr von Liebig (12 May 1803 – 20 April 1873) was a German scientist who made major contributions to agricultural and biological chemistry, and is considered one of the principal founders of organic chemistry. As a professor at the ...
and Wöhler in 1830 in their study of mellite ("honeystone"), but was characterized only in 1913, by Meyer and Steiner. Brodie also discovered in 1859 a fifth compound called
graphite oxide Graphite oxide (GO), formerly called graphitic oxide or graphitic acid, is a compound of carbon, oxygen, and hydrogen in variable ratios, obtained by treating graphite with strong oxidizers and acids for resolving of extra metals. The maximally ...
, consisting of carbon and oxygen in ratios varying between 2:1 and 3:1; but the nature and molecular structure of this substance remained unknown until a few years ago, when it was renamed
graphene oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...
and became a topic of research in
nanotechnology Nanotechnology, also shortened to nanotech, is the use of matter on an atomic, molecular, and supramolecular scale for industrial purposes. The earliest, widespread description of nanotechnology referred to the particular technological goal ...
. Notable examples of unstable or metastable oxides that were detected only in extreme situations are dicarbon monoxide radical (:C=C=O), carbon trioxide (CO3), carbon tetroxide (), carbon pentoxide (), carbon hexoxide () and 1,2-dioxetanedione (C2O4). Some of these reactive carbon oxides were detected within
molecular clouds A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
in the
interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar ...
by
rotational spectroscopy Rotational spectroscopy is concerned with the measurement of the energies of transitions between quantized rotational states of molecules in the gas phase. The spectra of polar molecules can be measured in absorption or emission by microwave sp ...
. H. M. Pickett E. A. Cohen B. J. Drouin J. C. Pearson (2003), ''Submillimeter, Millimeter, and Microwave Spectral Line Catalog''. NASA/
JPL The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA ...

Online version
accessed on 2009-07-11.
Many hypothetical oxocarbons have been studied by theoretical methods but have yet to be detected. Examples include oxalic anhydride (C2O3 or O=(C2O)=O),
ethylene dione Ethylene dione or ethylenedione, also called dicarbon dioxide, Carbon peroxide, ethenedione, or ethene-1,2-dione, is a chemical compound with the formula or . It is an oxide of carbon (an oxocarbon), and can be described as the carbon-carbon cov ...
(C2O2 or O=C=C=O) and other linear or cyclic polymers of carbon monoxide (-CO-)''n'' (
polyketone Polyketones are a family of high-performance thermoplastic polymers. The polar ketone groups in the polymer backbone of these materials gives rise to a strong attraction between polymer chains, which increases the material's melting point (255&nbs ...
s), and linear or cyclic polymers of carbon dioxide (-CO2-)''n'', such as the dimer 1,3-dioxetanedione (C2O4).


General structure

Normally, carbon is
tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of ...
, while oxygen is
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an ...
, and in most oxocarbons (as in most other carbon compounds) each carbon atom may be bound to four other atoms, while oxygen may be bound to at most two. Moreover, while carbon can connect to other carbons to form arbitrarily large chains or networks, chains of three or more oxygens are rarely if ever observed. Thus the known electrically neutral oxocarbons generally consist of one or more carbon skeletons (including cyclic and
aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satu ...
structures) connected and terminated by oxide (-O-, =O) or peroxide (-O-O-) groups. Carbon atoms with unsatisfied bonds are found in some oxides, such as the diradical C2O or :C=C=O; but these compounds are generally too reactive to be isolated in bulk. Loss or gain of electrons can result in monovalent negative oxygen (-), trivalent positive oxygen (≡), or trivalent negative carbon (≡). The last two are found in carbon monoxide, C≡O+. Negative oxygen occurs in most
oxocarbon anion In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers ''x'', ''y'', and ''n''. The most common oxocarbon anions are carbonate, , and oxalate, ...
s.


Linear carbon dioxides

One family of carbon oxides has the general formula CnO2, or O=(C=)''n''O — namely, a linear chain of carbon atoms, capped by oxygen atoms at both ends. The first members are * CO2 or O=C=O, the well-known carbon dioxide. * C2O2 or O=C=C=O, the unknown and extremely unstable
ethylene dione Ethylene dione or ethylenedione, also called dicarbon dioxide, Carbon peroxide, ethenedione, or ethene-1,2-dione, is a chemical compound with the formula or . It is an oxide of carbon (an oxocarbon), and can be described as the carbon-carbon cov ...
. * C3O2 or O=C=C=C=O, the metastable
carbon suboxide Carbon suboxide, or tricarbon dioxide, is an organic, oxygen-containing chemical compound with formula and structure . Its four cumulative double bonds make it a cumulene. It is one of the stable members of the series of linear oxocarbons , whi ...
or tricarbon dioxide. * C4O2 or O=C=C=C=C=O, tetracarbon dioxide or 1,2,3-Butatriene-1,4-dione * C5O2 or O=C=C=C=C=C=O,
pentacarbon dioxide Pentacarbon dioxide, officially penta-1,2,3,4-tetraene-1,5-dione, is an oxide of carbon (an oxocarbon) with formula C5O2 or O=C=C=C=C=C=O. The compound was described in 1988 by Günter Maier and others, who obtained it by pyrolysis of cyclohex ...
, stable in solution at room temp. and pure up to −90 °C. Some higher members of this family have been detected in trace amounts in low-pressure gas phase and/or cryogenic matrix experiments, specifically for ''n'' = 7Eastwood, Frank W. (1997), ''Gas Phase Pyrolytic Methods for the Preparation of Carbon-Hydrogen and Carbon-Hydrogen-Oxygen Compounds.''. In Yannick Vallée''Gas Phase Reactions in Organic Synthesis''.CRC Press. and ''n'' = 17, 19, and 21.


Linear carbon monoxides

Another family of oxocarbons are the linear carbon monoxides C''n''O. The first member, ordinary carbon monoxide CO, seems to be the only one that is practically stable in the pure state at room temperature (though it is not thermodynamically stable at
standard temperature and pressure Standard temperature and pressure (STP) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union o ...
, see
Boudouard reaction The Boudouard reaction, named after Octave Leopold Boudouard, is the redox reaction of a chemical equilibrium mixture of carbon monoxide and carbon dioxide at a given temperature. It is the disproportionation of carbon monoxide into carbon dioxide ...
).
Photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
of the linear carbon dioxides in a cryogenic matrix leads to loss of CO, resulting in detectable amounts of even-numbered monoxides such as C2O, C4O,Maier, Günter and Reisenauer, Hans Peter (2001) "Carbenes in Matrices: Specrospcopy, Structure, and Photochemical Behavior". In Udo H. Brinker (ed.), ''Advances in carbene chemistry'', p. 135. Elsevier. and C6O. The members up to ''n''=9 have also been obtained by electrical discharge on gaseous C3O2 diluted in argon. The first three members have been detected in interstellar space. When ''n'' is even, the molecules are believed to be in the triplet (
cumulene In organic chemistry, a cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is als ...
-like) state, with the atoms connected by double bonds and an unfilled orbital in the first carbon — as in :C=C=O, :C=C=C=C=O, and, in general, :(C=)''n''=O. When ''n'' is odd, the triplet structure is believed to
resonate Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscill ...
with a singlet (
acetylene Acetylene (systematic name: ethyne) is the chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in its pure f ...
-type) polar state with a negative charge on the carbon end and a positive one on the oxygen end, as in C≡C−C≡O+, C≡C−C≡C−C≡O+, and, in general, (C≡C−)(''n''−1)/2C≡O+. Carbon monoxide itself follows this pattern: its predominant form is believed to be C≡O+.


Radialene-type cyclic polyketones

Another family of oxocarbons that has attracted special attention are the cyclic radialene-type oxocarbons C''n''O''n'' or (CO)''n''. They can be regarded as cyclic polymers of carbon monoxide, or ''n''-fold ketones of ''n''-carbon
cycloalkanes In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure contain ...
. Carbon monoxide itself (CO) can be regarded as the first member. Theoretical studies indicate that ethylene dione (C2O2 or O=C=C=O) and cyclopropanetrione C3O3 do not exist. The next three members — C4O4, C5O5, and C6O6 — are theoretically possible, but are expected to be quite unstable, and so far they have been synthesized only in trace amounts. On the other hand, the
anions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
of these oxocarbons are quite stable, and some of them have been known since the 19th century. They are *C2O22−,
acetylenediol Acetylenediol, or ethynediol, is a chemical substance with formula HO−C≡C−OH (an ynol). It is the diol of acetylene. Acetylenediol is unstable in the condensed phase, although its tautomer glyoxal (CHO)2 is well known. Detection Acetylen ...
ate (Weiss and Büchner, 1963), *C3O32−, deltate (Eggerding and West, 1976), *C4O42−, squarate (Cohen and others, 1959), *C5O52−, croconate (Gmelin, 1825), and *C6O62−, rhodizonate (Heller, 1837). The cyclic oxide C6O6 also forms the stable anions of tetrahydroxy-1,4-benzoquinone (C6O64−) and
benzenehexol Benzenehexol, also called hexahydroxybenzene, is an organic compound with formula or . It is a six-fold phenol of benzene. The product is also called hexaphenol, but this name has been used also for other substances. Benzenehexol is a crystallin ...
(C6O66−), The
aromaticity In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satu ...
of these anions has been studied using theoretical methods.


New oxides

Many new stable or metastable oxides have been synthesized since the 1960s, such as: * C10O8,
benzoquinonetetracarboxylic dianhydride Benzoquinonetetracarboxylic dianhydride is an organic compound with formula (an oxide of carbon) which can be seen as the result of removing two molecules of water from benzoquinonetetracarboxylic acid. It is a red solid, stable in dry air up ...
(Hammond, 1963). * C6O6,
ethylenetetracarboxylic dianhydride Ethylenetetracarboxylic dianhydride is a chemical compound with formula , that can be seen as the twofold anhydride of ethylenetetracarboxylic acid . It has a bicyclic molecular structure consisting of two maleic anhydride rings fused by thei ...
, a stable isomer of cyclohexanehexone (Sauer and others, 1967). * C12O12 or C6(C2O4)3, hexahydroxybenzene trisoxalate (Verter and Dominic, 1967); stable as a
tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
solvate. * C10O10 or C6O2(C2O4)2,
tetrahydroxy-1,4-benzoquinone bisoxalate Tetrahydroxy-1,4-benzoquinone bisoxalate is a chemical compound, an oxide of carbon with formula . Its molecule consists of a 1,4-benzoquinone core with the four hydrogen atoms replaced by two oxalate groups. It can be seen as a fourfold ester o ...
(Verter and others, 1968); stable as a tetrahydrofuran solvate. * C8O8 or C6O2(CO3)2,
tetrahydroxy-1,4-benzoquinone biscarbonate Tetrahydroxy-1,4-benzoquinone biscarbonate is a chemical compound, an oxide of carbon with formula . Its molecule consists of a 1,4-benzoquinone core with the four hydrogen atoms replaced by two carbonate groups. It can be seen as a fourfold est ...
(Nallaiah, 1984); decomposes at about 45–53 °C. * C9O9 or C6(CO3)3, hexahydroxybenzene triscarbonate (Nallaiah, 1984); decomposes at about 45–53 °C. * C24O6, a cyclic trimer of the biradical 3,4-dialkynyl-3-cyclobutene1,2-dione -C≡C-(C4O2)-C≡C- (Rubin and others, 1990); * C32O8, a tetramer of 3,4-dialkynyl-3-cyclobutene1,2-dione (Rubin and others, 1990); * C4O6,
dioxane tetraketone Dioxane tetraketone (or 1,4-dioxane-2,3,5,6-tetrone) is an organic compound with the formula C4O6. It is an oxide of carbon (an oxocarbon), which can be viewed as the fourfold ketone of dioxane. It can also be viewed as the cyclic dimer of ox ...
or dimeric oxalic anhydride (Strazzolini and others, 1998); stable in Et2O at −30 °C, decomposes at 0 °C. * C12O6, hexaoxotricyclobutabenzene Many relatives of these oxides have been investigated theoretically, and some are expected to be stable, such as other carbonate and oxalate esters of tetrahydroxy-1,2-benzoquinone and of the rhodizonic, croconic, squaric, and deltic acids.


Polymeric carbon oxides

Carbon suboxide spontaneously polymerizes at room temperature into a carbon-oxygen
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...
, with 3:2 carbon:oxygen atomic ratio. The polymer is believed to be a linear chain of fused six-membered
lactone Lactones are cyclic carboxylic esters, containing a 1-oxacycloalkan-2-one structure (), or analogues having unsaturation or heteroatoms replacing one or more carbon atoms of the ring. Lactones are formed by intramolecular esterification of the co ...
rings, with a continuous carbon backbone of alternating single and double bonds. Physical measurements indicate that the mean number of units per molecule is about 5–6, depending on the formation temperature. Carbon monoxide compressed to 5
GPa Grading in education is the process of applying standardized measurements for varying levels of achievements in a course. Grades can be assigned as letters (usually A through F), as a range (for example, 1 to 6), as a percentage, or as a numbe ...
in a diamond anvil cell yields a somewhat similar reddish polymer with a slightly higher oxygen content, which is metastable at room conditions. It is believed that CO
disproportionates In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
in the cell to a mixture of CO2 and C3O2; the latter forms a polymer similar to the one described above (but with a more irregular structure), that traps some of the CO2 in its matrix. Another carbon-oxygen polymer, with C:O ratio 5:1 or higher, is the classical graphite oxide and its single-sheet version
graphene oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the ...
.


Fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...
oxides and ozonides

More than 20 oxides and ozonides of
fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ...
are known: * C60O (2 isomers) * C60O2 (6 isomers) * C60O3 (3 isomers) * C120O * C120O4 (4 isomers) * C70O * C140O and others.


See also

*
Oxocarbon anion In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers ''x'', ''y'', and ''n''. The most common oxocarbon anions are carbonate, , and oxalate, ...
* Pseudo-oxocarbon anion * Carbon nitride


References

{{Oxides