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In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula C
xOn−
y for some integers x, y, and n.
Contents
- Distributed charges and resonances
- Oxocarbon acids
- Neutralized species
- Reduced variants
- Acid anhydrides
- Hydrogenated anions
- Extreme cases
- List of oxocarbon anions
- References
The most common oxocarbon anions are carbonate, CO2−
3, and oxalate, C
2O2−
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.
Distributed charges and resonances
In many oxocarbon anions each of the extra electrons responsible for the negative electric charges behaves as if it were distributed over several atoms. Some of the electron pairs responsible for the covalent bonds also behave as if they were delocalized. These phenomena are often explained as a resonance between two or more conventional molecular structures that differ on the location of those charges and bonds. The carbonate ion, for example, is considered to have an "average" of three different structures:
so that each oxygen has the same negative charge equivalent to 2⁄3 of an electron, and each C–O bond has the same average valence of 4⁄3. This model accounts for the observed threefold symmetry of the anion.
Similarly, in a deprotonated carboxyl group –CO–
2, each oxygen is often assumed to have a charge of − 1⁄2 and each C–O bond to have valence 3⁄2, so the two oxygens are equivalent. The croconate anion C
5O2−
5 also has fivefold symmetry, that can be explained as the superposition of five states leading to a charge of − 2⁄5 on each oxygen. These resonances are believed to contribute to the stability of the anions.
Oxocarbon acids
An oxocarbon anion C
xOn−
y can be seen as the result of removing all protons from a corresponding acid CxHnOy. Carbonate CO2−
3, for example, can be seen as the anion of carbonic acid H2CO3. Sometimes the "acid" is actually an alcohol or other species; this is the case, for example, of acetylenediolate C
2O2−
2 that would yield acetylenediol C2H2O2. However, the anion is often more stable than the acid (as is the case for carbonate); and sometimes the acid is unknown or is expected to be extremely unstable (as is the case of methanetetracarboxylate C(COO−)4).
Neutralized species
Every oxocarbon anion C
xOn−
y can be matched in principle to the electrically neutral (or oxidized) variant CxOy, an oxocarbon (oxide of carbon) with the same composition and structure except for the negative charge. As a rule, however, these neutral oxocarbons are less stable than the corresponding anions. Thus, for example, the stable carbonate anion corresponds to the extremely unstable neutral carbon trioxide CO3; oxalate C
2O2−
4 correspond to the even less stable 1,2-dioxetanedione C2O4; and the stable croconate anion C
5O2−
5 corresponds to the neutral cyclopentanepentone C5O5, which has been detected only in trace amounts.
Reduced variants
Conversely, some oxocarbon anions can be reduced to yield other anions with the same structural formula but greater negative charge. Thus rhodizonate C
6O2−
6 can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C
6O4−
6 and then to benzenehexolate C
6O6−
6.
Acid anhydrides
An oxocarbon anion C
xOn−
y can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula CxOy− n⁄2; namely, the acid minus n⁄2 water molecules H2O. The standard example is the connection between carbonate CO2−
3 and carbon dioxide CO2. The correspondence is not always well-defined since there may be several ways of performing this formal dehydration, including joining two or more anions to make an oligomer or polymer. Unlike neutralization, this formal dehydration sometimes yields fairly stable oxocarbons, such as mellitic anhydride C12O9 from mellitate C
12O6−
12 via mellitic acid C12H6O12
Hydrogenated anions
For each oxocarbon anion C
xOn−
y there are in principle n−1 partially hydrogenated anions with formulas H
kC
xO(n−k)−
y, where k ranges from 1 to n−1. These anions are generally indicated by the prefixes "hydrogen"-, "dihydrogen"-, "trihydrogen"-, etc. Some of them, however, have special names: hydrogencarbonate HCO−
3 is commonly called bicarbonate, and hydrogenoxalate HC
2O−
4 is known as binoxalate.
The hydrogenated anions may be stable even if the fully protonated acid is not (as is the case of bicarbonate).
Extreme cases
The carbide anions, such as acetylide C2−
2 and methanide C4−, could be seen as extreme cases of oxocarbon anions C
xOn−
y, with y equal to zero. The same could be said of oxygen-only anions such as oxide O2−, superoxide O−
2, peroxide, O2−
2, and ozonide O−
3.
List of oxocarbon anions
Here is an incomplete list of the known or conjectured oxocarbon anions
Several other oxocarbon anions have been detected in trace amounts, such as C
6O−
6, a singly ionized version of rhodizonate.