Oxocarbon anion

 ( Carbon Oxyanions ) 

In chemistry, an oxocarbon anion is a anion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers x, y, and n.

The most common oxocarbon anions are carbonate, , and oxalate, . There are 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 and , that have a fleeting existence during some chemical reactions; and many hypothetical species, like , that have been the subject of theoretical studies but have yet to be observed.

Stable oxocarbon anions form salts with a large variety of . Unstable anions may persist in very rarefied gaseous state, such as in interstellar clouds. Most oxocarbon anions have corresponding Functional group in organic chemistry, whose compounds are usually esters. Thus, for example, the oxalate moiety occurs in the ester dimethyl oxalate .

---

Electronic structure of the carbonate ion The carbonate ion has a trigonal planar structure, point group D_3h. The three C-O bonds have the same length of 136 pm and the 3 O-C-O angles are 120°. The carbon atom has 4 pairs of valence electrons, which shows that the molecule obeys the octet rule. This is one factor that contributes to the high stability of the ion, which occurs in rocks such as limestone. The electronic structure is described by two main theories which are used to show how the 4 electron pairs are distributed in a molecule that only has 3 C-O bonds.

With valence bond theory the electronic structure of the carbonate ion is a resonance hybrid of 3 canonical forms.

In each canonical form there are two single bonds one double bond. The three canonical forms contribute equally to the resonance hybrid, so the three bond C-O bonds have the same length. With molecular orbital theory the 3-fold axis is designated as the z axis of the molecule. Three σ bonds are formed overlap of the s, p_x and p_y orbitals on the carbon atom with a p orbital on each oxygen atom. In addition, a delocalized π bond is made by overlap of the p_z orbital on the carbon atom with the p_z orbital on each oxygen atom which is perpendicular to the plane of the molecule.

Note that the same bonding schemes may be applied the nitrate ion, NO₃⁻, which is isoelectronic with the carbonate ion.

Similarly, the two-fold symmetrical structure of a carboxylate group,, may be described as a resonance hybrid of two canonical forms in valence bond theory, or with 2 σ bonds and a delocalized π bond in molecular orbital theory.

---

Related compounds

---

Oxocarbon acids An oxocarbon anion can be seen as the result of removing all from a corresponding acid C _xH _nO _y. Carbonate , 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 that would yield acetylenediol C₂H₂O₂. However, the anion is often more stable than the acid (as is the case for carbonate); "Infrared and mass spectral studies of proton irradiated H₂O + CO₂ ice: evidence for carbonic acid", by Moore, M. H.; Khanna, R. K. 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 can be matched in principle to the electrically neutral (or oxidation) variant C _xO _y, 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 correspond to the even less stable 1,2-dioxetanedione C₂O₄;

and the stable croconic acid anion 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 rhodizonic acid can be reduced to the tetrahydroxybenzoquinone (THBQ) anion and then to .

 Haiyan Chen, Michel Armand, Matthieu Courty, Meng Jiang, Clare P. Grey, Franck Dolhem, Jean-Marie Tarascon, and Philippe Poizot (2009), "Lithium Salt of Tetrahydroxybenzoquinone: Toward the Development of a Sustainable Li-Ion Battery"  ''J. Am. Chem. Soc.'', '''131'''(25), pp. 8984–8988
     

---

Acid anhydrides

An oxocarbon anion can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula C _xO _y−; namely, the acid minus water molecules H₂O. The standard example is the connection between carbonate 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 via mellitic acid C₁₂H₆O₁₂

 J. Liebig, F. Wöhler (1830), "Ueber die Zusammensetzung der Honigsteinsäure" ''Poggendorfs Annalen der Physik und Chemie'', vol. 94, Issue 2, pp.161–164. [https://books.google.com/books?id=ZyUAAAAAMAAJ&pg=PA161 Online version] accessed on 2009-07-08.
     

---

Hydrogenated anions

For each oxocarbon anion there are in principle n−1 partially hydrogenated anions with formulas , 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 is commonly called bicarbonate, and hydrogenoxalate is known as binoxalate.

The hydrogenated anions may be stable even if the fully protonated acid is not (as is the case of bicarbonate).

---

List of oxocarbon anions

Here is an incomplete list of the known or conjectured oxocarbon anions

	:	carbonite	C(OH)2 (carbonous acid)	carbon monoxide	carbon dioxide
		carbonate	carbonic acid	carbon dioxide	carbon trioxide
		peroxocarbonate		carbon trioxide	carbon tetroxide
		orthocarbonate	C(OH)4 methanetetrol	carbon dioxide	carbon tetroxide
		acetylenediolate	acetylenediol		ethylene dione
		oxalate	oxalic acid	oxalic anhydride, C4O6	C2O4
		dicarbonate	dicarbonic acid	C2O4
		peroxodicarbonate
		deltate	deltic acid		C3O3
		mesoxalate	mesoxalic acid
		acetylenedicarboxylate	C4H2O4
		squarate	squaric acid		C4O4
		dioxosuccinate	C4H2O6
		croconate	croconic acid		C5O5
		methanetetracarboxylate	C5H4O8
		rhodizonate	rhodizonic acid		C6O6
		benzoquinonetetraolate; THBQ anion	(CO)2(COH)4 THBQ		C6O6
		benzenehexolate	benzenehexol		C6O6
		ethylenetetracarboxylate	C6H4O8	C6O6
		furantetracarboxylate	C8H4O9	C8O7
		benzoquinonetetracarboxylate
		mellitate	mellitic acid	C12O9

Several other oxocarbon anions have been detected in trace amounts, such as , a singly ionized version of rhodizonate.

 Richard B. Wyrwas and Caroline Chick Jarrold (2006), "Production of  from Oligomerization of CO on Molybdenum Anions". ''J. Am. Chem. Soc.'' volume 128 issue 42, pages 13688–13689.
     

---

See also

• Oxocarbon
• Silicate
• Sodium percarbonate (actually a carbonate perhydrate)

Categories: Carbon Oxyanions, Oxocarbons

Post Comment