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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, 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 element ...
consisting only of
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
and
oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
. The simplest and most common oxocarbons are
carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
(CO) and
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
(). Many other stable (practically if not thermodynamically) or
metastable In chemistry and physics, metastability is 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 is onl ...
oxides of carbon are known, but they are rarely encountered, such as carbon suboxide ( or ) and mellitic anhydride (). 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 is 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 is onl ...
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. Graphene oxide and other stable
polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
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 type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
of foods such as
beer Beer is an alcoholic beverage produced by the brewing and fermentation of starches from cereal grain—most commonly malted barley, although wheat, maize (corn), rice, and oats are also used. The grain is mashed to convert starch in the ...
and
bread Bread is a baked food product made from water, flour, and often yeast. It is a staple food across the world, particularly in Europe and the Middle East. Throughout recorded history and around the world, it has been an important part of many cu ...
. 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 and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...
of
iron Iron is a chemical element; it has symbol Fe () 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, forming much of Earth's o ...
from its
ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...
s. Like the dioxide, it was described and studied in the West by various
alchemist Alchemy (from the Arabic word , ) is an ancient branch of natural philosophy, a philosophical and protoscientific tradition that was historically practised in China, India, the Muslim world, and Europe. In its Western form, alchemy is first ...
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 (C12O9), was apparently obtained by Liebig 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, 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 and became a topic of research in
nanotechnology Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...
. 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 of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...
in the
interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...
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 rotational spectrum (power spectral density vs. rotational frequency) of chemical pola ...
. H. M. Pickett E. A. Cohen B. J. Drouin J. C. Pearson (2003), ''Submillimeter, Millimeter, and Microwave Spectral Line Catalog''.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
/ JPL
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 (C2O2 or O=C=C=O) and other linear or cyclic polymers of carbon monoxide (-CO-)''n'' ( polyketones), and linear or cyclic polymers of carbon dioxide (-CO2-)''n'', such as the dimer 1,3-dioxetanedione (C2O4).


General structure

Normally, carbon is tetravalent, while oxygen is
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemica ...
, 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 organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
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 anions.


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 Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
. * C2O2 or O=C=C=O, the unknown and extremely unstable ethylene dione. * C3O2 or O=C=C=C=O, the metastable carbon suboxide 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, 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) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used ...
, see Boudouard reaction).
Photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
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 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 also called simply ''cumu ...
-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 with a singlet (
acetylene Acetylene (Chemical nomenclature, systematic name: ethyne) is a 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 u ...
-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. 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−, acetylenediolate (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 (C6O66−), The
aromaticity In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugation alone. The e ...
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 (Hammond, 1963). * C6O6, ethylenetetracarboxylic dianhydride, 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 ...
solvate. * C10O10 or C6O2(C2O4)2, tetrahydroxy-1,4-benzoquinone bisoxalate (Verter and others, 1968); stable as a tetrahydrofuran solvate. * C8O8 or C6O2(CO3)2, tetrahydroxy-1,4-benzoquinone biscarbonate (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-cyclobutene-1,2-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 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 () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
, 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. They are derived from the corresponding hydroxycarboxylic acids by esterification. They can be saturated or unsaturated. Lactones are formed by lactonization, the intramolecular esterification of the corresp ...
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 in a
diamond anvil cell A diamond anvil cell (DAC) is a high-pressure device used in geology, engineering, and materials science experiments. It permits the compression of a small (sub- millimeter-sized) piece of material to extreme pressures, typically up to around 1 ...
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 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.


Fullerene oxides and ozonides

More than 20 oxides and ozonides of fullerene are known: * C60O (2 isomers) * C60O2 (6 isomers) * C60O3 (3 isomers) * C120O * C120O4 (4 isomers) * C70O * C140O and others.


See also

* Pseudo-oxocarbon anion * Carbon nitride


References

{{Oxides