Oxocarbon
In
CO Carbon monoxide |
CO2 Carbon dioxide |
C3O2 Carbon suboxide |
C12O9 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
Overview
Carbon dioxide (CO2) occurs widely in nature, and was incidentally produced by humans since pre-historical times, by breathing, the
. 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
Carbon suboxide was discovered by Benjamin Brodie in 1873, by passing electric current through carbon dioxide.[5]
The fourth "classical" oxide,
Brodie also discovered in 1859 a fifth compound called
Notable examples of unstable or metastable oxides that were detected only in extreme situations are
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)[17] and other linear or cyclic polymers of carbon monoxide (-CO-)n (polyketones),[18] and linear or cyclic polymers of carbon dioxide (-CO2-)n, such as the dimer 1,3-dioxetanedione (C2O4).[19]
C2O3 Oxalic anhydride |
C2O2 Ethylene dione |
C2O4 1,3-Dioxetane- dione |
General structure
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.[20] 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+.[21] 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 unknown and extremely unstable ethylene dione.[17]
- 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[22]
- C5O2 or O=C=C=C=C=C=O, pentacarbon dioxide,[23] stable in solution at room temp. and pure up to −90 °C.[24]
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 = 7[24]: p.97 and n = 17, 19, and 21.[25]: p.95
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 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,[20] and C6O.[24] The members up to n=9 have also been obtained by electrical discharge on gaseous C3O2 diluted in argon.[26] The first three members have been detected in interstellar space.[26]
When n is even, the molecules are believed to be in the
Radialene-type cyclic polyketones
Another family of oxocarbons that has attracted special attention are the cyclic
(CO)2 Ethylene dione |
(CO)3 Cyclopropane- trione |
(CO)4 Cyclobutane- tetrone |
(CO)5 Cyclopentane- pentone |
(CO)6 Cyclohexane- hexone |
On the other hand, the
- C2O22−, acetylenediolate (Weiss and Büchner, 1963),[30]
- C3O32−, deltate (Eggerding and West, 1976),[31][32]
- C4O42−, squarate (Cohen and others, 1959),[33]
- C5O52−, croconate (Gmelin, 1825),[34] and
- C6O62−, rhodizonate (Heller, 1837).[35][36]
The cyclic oxide C6O6 also forms the stable anions of tetrahydroxy-1,4-benzoquinone (C6O64−) and benzenehexol (C6O66−),[37] The aromaticity of these anions has been studied using theoretical methods.[38][39]
New oxides
Many new stable or metastable oxides have been synthesized since the 1960s, such as:
- C10O8, benzoquinonetetracarboxylic dianhydride (Hammond, 1963).[40]
- C6O6, ethylenetetracarboxylic dianhydride, a stable isomer of cyclohexanehexone (Sauer and others, 1967).[41]
- C12O12 or C6(C2O4)3, hexahydroxybenzene trisoxalate (Verter and Dominic, 1967); stable as a tetrahydrofuran solvate.[42]
- C10O10 or C6O2(C2O4)2, tetrahydroxy-1,4-benzoquinone bisoxalate (Verter and others, 1968); stable as a tetrahydrofuran solvate.[43]
- C8O8 or C6O2(CO3)2, tetrahydroxy-1,4-benzoquinone biscarbonate (Nallaiah, 1984); decomposes at about 45–53 °C.[44]
- C9O9 or C6(CO3)3, hexahydroxybenzene triscarbonate (Nallaiah, 1984); decomposes at about 45–53 °C.[44]
- C24O6, a cyclic trimer of the biradical 3,4-dialkynyl-3-cyclobutene-1,2-dione -C≡C-(C4O2)-C≡C- (Rubin and others, 1990);[45]
- C32O8, a tetramer of 3,4-dialkynyl-3-cyclobutene1,2-dione (Rubin and others, 1990);[45]
- C4O6,
- C12O6, hexaoxotricyclobutabenzene[47][48]
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.[18]
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.[4][49]
Terminating and repeating units of polymeric C3O2.[4] |
Oligomers of C3O2 with 3 to 6 units.[4] |
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.[50][51]
Another carbon-oxygen polymer, with C:O ratio 5:1 or higher, is the classical graphite oxide
Fullerene oxides and ozonides
More than 20 oxides and ozonides of fullerene are known:[52]
- C60O (2 isomers)
- C60O2 (6 isomers)
- C60O3 (3 isomers)
- C120O
- C120O4 (4 isomers)
- C70O
- C140O
and others.
See also
References
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