The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (CO2) with IUPAC names carbon(II) oxide and carbon(IV) oxide respectively. Many other stable (practically if not thermodynamically) or metastable oxides of carbon are known, but they are rarely encountered, such as carbon suboxide (C3O2 or O=C=C=C=O) and mellitic anhydride (C12O9).
While textbooks will often list only the first three, and rarely the fourth, a large number of 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 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 can exist only in the gas phase or under matrix isolation conditions.
The inventory of oxocarbons appears to be steadily growing. The existence of graphene oxide and of other stable polymeric carbon oxides with unbounded molecular structures suggests that many more remain to be discovered.
Carbon dioxide (CO2) occurs widely in nature, and was incidentally manufactured by humans since pre-historical times, by the combustion of carbon-containing substances and fermentation of foods such as beer and bread. 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 of iron from its ores. Like the dioxide, it was described and studied in the West by various alchemists and chemists since the Middle Ages. Its true composition was discovered by William Cruikshank in 1800.
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.
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 (CO
4), carbon pentoxide (CO
5), carbon hexoxide (CO
6) and 1,2-dioxetanedione (C2O4). Some of these reactive carbon oxides were detected within molecular clouds in the interstellar medium by rotational spectroscopy.
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).
Normally, carbon is tetravalent, while oxygen is divalent, 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 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 (-O−
), trivalent positive oxygen (≡O+
), or trivalent negative carbon (≡C−
). The last two are found in carbon monoxide, −C≡O+. Negative oxygen occurs in most oxocarbon anions.
One family of carbon oxides has the general formula CnO2, or O=(C=)nO — namely, a linear chain of carbon atoms, capped by oxygen atoms at both ends. The first members are
Another family of oxocarbons are the linear carbon monoxides CnO. 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, see Boudouard reaction). Photolysis 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, 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-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-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+.
Another family of oxocarbons that has attracted special attention are the cyclic radialene-type oxocarbons CnOn 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.
The cyclic oxide C6O6 also forms the stable anions of tetrahydroxy-1,4-benzoquinone (C6O64−) and benzenehexol (C6O66−), The aromaticity of these anions has been studied using theoretical methods.
Many new stable or metastable oxides have been synthesized since the 1960s, such as:
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.
Carbon suboxide spontaneously polymerizes at room temperature into a carbon-oxygen polymer, with 3:2 carbon:oxygen atomic ratio. The polymer is believed to be a linear chain of fused six-membered lactone 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.
|Terminating and repeating units of polymeric C3O2.|
|Oligomers of C3O2 with 3 to 6 units.|
Carbon monoxide compressed to 5 GPa 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 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.
1,2-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C10N4O2−2 or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C4O2−4 through the replacement of two adjacent oxygen atoms by dicyanomethylene groups =C(−C≡N)2.
The anion can be obtained by reacting squaric acid with n-butanol to obtain the diester 1,2-dibutyl squarate (an oily orange liquid) and treating the latter with metallic sodium and malononitrile (N≡C−)2CH2 to give the trihydrated disodium salt 2Na+·C10N4O2−2·3H2O, a yellow water-soluble solid. The hydrated salt loses the water below 100 °C, but the resulting anhydrous salt is stable up to 400 °C. Reaction of the sodium salt with the chlorides of other cations in ethanol affords the following salts:
dipotassium 2K+·K2C10N4O2−2, anhydrous, yellow, stable to 300 °C
dirubidium 2Rb+·Rb2C10N4O2−2, anhydrous, brown, stable to 300 °C
magnesium sodium chloride, Mg2+·Na+·Cl−·C10N4O2−2·4 1⁄2H2O, dark yellow, dehydrates at 60–106 °C, stable to 461 °C
calcium disodium, 2Na+·Ca2+·2C10N4O2−2·9H2O, yellow, dehydrates at 50–90 °C, stable to 178 °C
barium, Ba2+·C10N4O2−2·2H2O, yellow, dehydrates at 87 °C, stable to 337 °C
tetra-n-butylammonium, 2(C4H5)4N+·C10N4O2−2·H2O, yellow, dehydrates at 145 °C, stable to 323 °CNuclear magnetic resonance shows that the aromatic character of the squarate core is retained.1,2-Dioxetanedione
The chemical compound 1,2-dioxetanedione, or 1,2-dioxacyclobutane-3,4-dione, often called peroxyacid ester, is an unstable oxide of carbon (an oxocarbon) with formula C2O4. It can be viewed as a double ketone of 1,2-dioxetane (1,2-dioxacyclobutane), or a cyclic dimer of carbon dioxide.In ordinary conditions, it quickly decomposes to carbon dioxide (CO2) even at 180 K (−93.1 °C), but can be detected by mass spectrometry and other techniques.1,2-Dioxetanedione is an intermediate in the chemoluminescent reactions used in glowsticks. The decomposition proceeds via a paramagnetic oxalate biradical intermediate.Recently it has been found that a high-energy intermediate in one of these reactions (between oxalyl chloride and hydrogen peroxide in ethyl acetate), which is presumed to be 1,2-dioxetanedione, can accumulate in solution at room temperature (up to a few micromoles at least), provided that the activating dye and all traces of metals and other reducing agents are removed from the system, and the reactions are carried out in an inert atmosphere.1,3-Bis(dicyanomethylene)squarate
1,3-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C10N4O2−2 or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C4O2−4 through the replacement of two opposite oxygen atoms by dicyanomethylene groups =C(−C≡N)2.
The anion can be obtained by reacting squaric acid with aniline to form the diester 1,3-dianiline squarate (a yellow solid), before treating the diester with malononitrile (N≡C−)2CH2 and sodium ethoxide to give the disodium tetrahydrate salt 2Na+·C10N4O2−2·4H2O, an orange water-soluble solid. The hydrated salt loses the water below 100 °C, but the resulting anhydrous salt is stable up to 400 °C. Reaction of the sodium salt with salts of other cations in ethanol affords the following salts:
dipotassium sodium chloride 2K+·Na+·Cl−·K2C10N4O2−2·1⁄2CH3CN, orange
rubidium sodium chloride 7Rb+·Na+·2Cl−·3Rb2C10N4O2−2·CH3CH2OH, orange, loses 1 ethanol at 96 °C, stable to 361 °C
magnesium disodium nitrate, Mg2+·2Na+C10N4O2−2·NO−3·6H2O·CH3CH2OH, orange, loses 1 ethanol and 6 H2O at 78 °C, stable to 482 °C
calcium, Ca2+·C10N4O2−2·6H2O, purple, dehydrates at 63–102 °C, stable to 468 °C
barium, Ba2+·C10N4O2−2·4H2O, orange, dehydrates at 71–96 °C, stable to 457 °C
tetra-n-butylammonium sodium, 2(C4H9)4N+·2Na+·2Cl−·2C10N4O2−2·CH3CH2OH, orange, loses 1 ethanol and 2 tetrabutylammonium at 111 °C, stable to 238 °CNuclear magnetic resonance shows that the aromatic character of the squarate core is retained.2-(Dicyanomethylene)croconate
2-(Dicyanomethylene)croconate is a divalent anion with chemical formula C8N2O2−4 or ((N≡C−)2C=)(C5O4)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of one oxygen atom by a dicyanomethylene group =C(−C≡N)2.
The anion was synthesized and characterized by A. Fatiadi in 1980, by hydrolysis of croconate violet treated with potassium hydroxide. It gives an orange solution in water.Carbon trioxide
Carbon trioxide (CO3) is an unstable oxide of carbon (an oxocarbon). Three possible isomers of carbon trioxide, with molecular symmetry point groups Cs, D3h, and C2v, have been most studied by theoretical methods, and the C2v state has been shown to be the ground state of the molecule. Carbon trioxide should not be confused with the stable carbonate ion (CO32−).
Carbon trioxide can be produced, for example, in the drift zone of a negative corona discharge by reactions between carbon dioxide (CO2) and the atomic oxygen (O) created from molecular oxygen by free electrons in the plasma. Another reported method is photolysis of ozone O3 dissolved in liquid CO2, or in CO2/SF6 mixtures at -45 °C, irradiated with light of 253.7 nm. The formation of CO3 is inferred but it appears to decay spontaneously by the route 2CO3 → 2CO2 + O2 with a lifetime much shorter than 1 minute. Carbon trioxide can be made by blowing ozone at dry ice (solid CO2), and it has also been detected in reactions between carbon monoxide (CO) and molecular oxygen (O2). Along with the ground state C2v isomer, the first spectroscopic detection of the D3h isomer was in electron-irradiated ices of carbon dioxide.Croconate blue
Croconate blue or 1,2,3-tris(dicyanomethylene)croconate is a divalent anion with chemical formula C14N6O2−2 or ((N≡C−)2C=)3(C5O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of three oxygen atoms by dicyanomethylene groups =C(−C≡N)2. The term Croconate Blue as a dye name specifically refers to the dipotassium salt K2C14N6O2.Croconate violet
Croconate violet or 1,3-bis(dicyanomethylene)croconate is a divalent anion with chemical formula C11N4O2−3 or ((N≡C−)2C=)2(C5O3)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C5O2−5 through the replacement of two oxygen atoms by dicyanomethylene groups =C(−C≡N)2. Its systematic name is 3,5-bis(dicyanomethylene)-1,2,4-trionate. The term croconate violet as a dye name specifically refers to the dipotassium salt K2C11N4O3.Cyclohexanehexone
Cyclohexanehexone, also known as hexaketocyclohexane and triquinoyl, is an organic compound with formula C6O6, the sixfold ketone of cyclohexane. It is an oxide of carbon (an oxocarbon), a hexamer of carbon monoxide.
The compound is expected to be highly unstable, unlike the cyclohexanehexathione analog, and as of 1999 had only been observed as an ionized fragment during mass spectrometry studies.Cyclopentanepentone
Cyclopentanepentone, also known as leuconic acid, is a hypothetical organic compound with formula C5O5, the fivefold ketone of cyclopentane. It would be an oxide of carbon (an oxocarbon), indeed a pentamer of carbon monoxide.
As of 2000, the compound had yet to be synthesized in bulk, but there have been reports of trace synthesis.Dicarbon monoxide
Dicarbon monoxide (C2O) is a molecule that contains two carbon atoms and one oxygen atom. It is a linear molecule that, because of its simplicity, is of interest in a variety of areas. It is, however, so extremely reactive that it is not encountered in everyday life. It is classified as a cumulene and an oxocarbon.Dicarbonate
In organic chemistry, a dicarbonate, also known as a pyrocarbonate, is a compound containing the divalent [−O−(C=O)−O−(C=O)−O−] or −C2O5− functional group, which consists of two carbonate groups sharing an oxygen atom. These compounds can be viewed as double esters of a hypothetical dicarbonic acid, H2C2O5 or HO−(C=O)−O−(C=O)−OH. Two important examples are dimethyl dicarbonate H3C−C2O5−CH3 and di-tert-butyl dicarbonate (H3C−)3C−C2O5−C(−CH3)3.
It is one of the oxocarbon anions, consisting solely of oxygen and carbon. Dicarbonate salts are apparently unstable but may have a fleeting existence in carbonate solutions.The term "dicarbonate" is sometimes used erroneously to refer to bicarbonate, the common name of the hydrogencarbonate anion HCO−3 or organic group the ROCO2H.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 oxiranedione (C2O3), the hypothetical anhydride of oxalic acid.
In 1998, Paolo Strazzolini and others synthesized this compound by reacting oxalyl chloride (COCl)2 or the bromide (COBr)2 with a suspension of silver oxalate (Ag2C2O4) in diethyl ether at −15 °C, followed by evaporation of the solvent at low temperature and pressure. The substance is stable when dissolved in ether and trichloromethane at −30 °C, but decomposes into a 1:1 mixture of carbon monoxide (CO) and carbon dioxide (CO2) upon heating to 0 °C. The stability and conformation of the molecule were also analyzed by theoretical methods.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 CO2, CO, and C3O2, is one of the only four that are reasonably stable under standard conditions. It is a white sublimable solid, apparently obtained by Justus Liebig and Friedrich Wöhler in 1830 in their study of mellite ("honey stone") and has the empirical formula C4O3. The substance was properly characterized in 1913 by H. Meyer and K. Steiner. It retains the aromatic character of the benzene ring.Oxalic anhydride
Oxalic anhydride or ethanedioic anhydride, also called oxiranedione, is a hypothetical organic compound with the formula C2O3, which can be viewed as the anhydride of oxalic acid or the two-fold ketone of ethylene oxide. It is an oxide of carbon (an oxocarbon).
The simple compound apparently has yet to be observed (as of 2009). In 1998, however, Paolo Strazzolini and others have claimed the synthesis of dioxane tetraketone (C4O6), which can be viewed as the cyclic dimer of oxalic anhydride.It has been conjectured to be a fleeting intermediate in the thermal decomposition of certain oxalates and certain chemoluminescent reactions of oxalyl chloride.Oxocarbon anion
In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula CxOn−y for some integers x, y, and n.
The most common oxocarbon anions are carbonate, CO2−3, and oxalate, C2O2−4. There is however a large number of stable anions in this class, including several ones that have research or industrial use. There are also many unstable anions, like CO−2 and CO−4, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO4−4, that have been the subject of theoretical studies but have yet to be observed.
Stable oxocarbon anions form salts with a large variety of cations. Unstable anions may persist in very rarefied gaseous state, such as in interstellar clouds. Most oxocarbon anions have corresponding moieties in organic chemistry, whose compounds are usually esters. Thus, for example, the oxalate moiety [–O–(C=O)2–O–] occurs in the ester dimethyl oxalate H3C–O–(C=O)2–O–CH3.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 cyclohexane-1,3,5-trione (phloroglucin, the tautomeric form of phloroglucinol). It has also been obtained by flash vapor pyrolysis of 2,4,6-tris(diazo)cyclohexane-1,3,5-trione (C6N6O3). It is stable at room temperature in solution. The pure compound is stable up to −96 °C, at which point it polymerizes.Peroxydicarbonate
In chemistry, peroxydicarbonate (sometimes peroxodicarbonate) is a divalent anion with formula C2O2−6. It is one of the oxocarbon anions, which consist solely of carbon and oxygen. Its molecular structure can be viewed as two carbonate anions joined so as to form a peroxide bridge –O–O–.
The anion is formed, together with peroxocarbonate CO2−4, at the negative electrode during electrolysis of molten lithium carbonate. The anion can also be obtained by electrolysis of a saturated solution of rubidium carbonate in water.Potassium peroxydicarbonate K2C2O6 was obtained by Constam and von Hansen in 1895; its crystal structure was determined only in 2002. It too can be obtained by electrolysis of a saturated potassium carbonate solution at −20 °C. It is a light blue crystalline solid that decomposes at 141 °C, releasing oxygen and carbon dioxide, and decomposes slowly at lower temperatures.Rubidium peroxodicarbonate is a light blue crystalline solid that decomposes at 424 K (151 °C). Its structure was published in 2003. In both salts, each of the two carbonate units is planar. In the rubidium salt the whole molecule is planar, whereas in the potassium salt the two units lie on different and nearly perpendicular planes, both of which contain the O–O bond.Pseudo-oxocarbon anion
In chemistry, the term pseudo-oxocarbon anion is used to refer to a negative ion that is conceptually derived from an oxocarbon anion through replacement of one or more of the basic oxygen atoms by chemically similar elements or functional groups, such as sulfur (S), selenium (Se), or dicyanomethylene (=C(CN)2).
Typical examples are the anions 2-(Dicyanomethylene)croconate, croconate violet, and croconate blue, derived from the croconate anion C5O2−5 by replacing one, two, or three oxygen atoms by dicyanomethylene groups:
These anions retain many of the properties of the parent, including the delocalized bond in the ring and the delocalized charge in the atoms attached to the ring. Similar anions can be obtained from squarate C4O2−4.Tricarbon monoxide
Tricarbon monoxide C3O is a reactive radical oxocarbon molecule found in space, and which can be made as a transient substance in the laboratory. It can be trapped in an inert gas matrix or made as a short lived gas. C3O can be classified as a ketene or an oxocumulene a kind of heterocumulene.
|Compounds derived from oxides|
|Oxides and related|
|Mixed oxidation states|
|+1 oxidation state|
|+2 oxidation state|
|+3 oxidation state|
|+4 oxidation state|
|+5 oxidation state|
|+6 oxidation state|
|+7 oxidation state|
|+8 oxidation state|
Oxides are sorted by oxidation state. Category:Oxides