The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (CO2). Many other stable 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 sylvestre ("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.
Carbon suboxide was discovered by 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.
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) and the trimer 1,3,5-trioxanetrione (C3O6).
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.
Linear carbon dioxides
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
- CO2 or O=C=O, the well-known carbon dioxide.
- C2O2 or O=C=C=O, the 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.
Linear carbon monoxides
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 stable in the pure state at room temperature. 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+.
Radialene-type cyclic polyketones
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.
- 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 of these anions has been studied using theoretical methods.
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 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-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 or dimeric oxalic anhydride (Strazzolini and others, 1998); stable in Et2O at −30 °C, decomposes at 0 °C.
- C12O6, hexaoxotricyclobutabenzene (Hamura and others, 2006)
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, 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.
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