Croconic acid
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| Names | |
|---|---|
| Other names
1,2-dihydroxycyclopentene-3,4,5-trione
4,5-dihydroxy-4-cyclopentene-1,2,3-trione Crocic acid | |
| Identifiers | |
3D model (JSmol)
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| ECHA InfoCard | 100.201.686 |
CompTox Dashboard (EPA)
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| Properties | |
| C5H2O5 | |
| Molar mass | 142.07 |
| Melting point | > 300°C (dec.) |
| Acidity (pKa) | 0.80, 2.24 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Croconic acid or 1,2-dihydroxycyclopentenetrione is a chemical compound with formula C5H2O5 or (C=O)3(COH)2. It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms. It is sensitive to light[1], soluble in water and ethanol[2] and forms yellow crystals that decompose at 212°C.[3]
The compound is acidic and loses the hydrogen cations H+ from the hydroxyls (pK1 = 0.80 ± 0.08 and pK2 = 2.24 ± 0.01 at 25°C). [4][5] The resulting anions, hydrogencroconate C5HO5− [1] and croconate C5O52− are also quite stable. The croconate ion, in particular, is aromatic and symmetric, as the double bond and the negative charges become delocalized over the five CO units. The lithium, sodium and potassium croconates crystallize from water as dihydrates[6] but the orange potassium salt can be dehydrated to form a monohydrate. [1][4] The croconates of ammonium, rubidium and caesium crystallize in the anhydrous form.[6] Salts of barium, lead, silver, etc. are also known.[1]
Croconic acid also forms esters such as dimethyl croconate.
History
Croconic acid and potassium croconate dihydrate were discovered by Leopold Gmelin in 1825, who named the compounds from Greek τό χρόχον meaning "saffron" or "egg yolk".[6] The structure of ammonium croconate was determined by Baenziger et al. in 1964. The structure of K2C5O5 . 2H2O was determined by J. D. Dunitz in 2001 [7]
Structure
In the solid-state, croconic acid has a peculiar strcuture consisiting of pleated strips, each "page" of the strip being a planar ring of 4 molecules of C5O5H2 held together by hydrogen bonds.[6]
In the solid alkali metal salts, the croconate anions and the alkali cations form parallel columns.[6] In the mixed salt K3(HC5O5)(C5O5)·2H2O, which formally contains both the croconate dianion C5O52− and the hydrogencroconate monoanion (HC5O5−), the hydrogen is shared equally by two adjacent croconate units.[6]
References
- ^ a b c d Kiyoyuki Yamada, Nobuhisa Mizuno and Yoshimasa Hirata (1958), Structure of Croconic Acid. Bulletin of the Chemical Society of Japan volume 31, issue 5, pp. 543-549 doi:10.1246/bcsj.31.543.
- ^ William Allen Miller (1868), Elements of chemistry: theoretical and practical, 4th edition. 884 pages. Longmans
- ^ Edward Turner, Elements of chemistry
- ^ a b Lowell M. Schwmtr Robert I. Gelb, and Janet 0. Yardley (1975), Aqueous Dissociation of Croconic Acid. J. Phys. Chem., volume 79 issue 21, pp 2246–2251. doi:10.1021/j100588a009
- ^ Robert I. Gelb, Lowell M. Schwartz, Daniel A. Laufer, Janet O. Yardley (1977), The structure of aqueous croconic acid. J. Phys. Chem., volume 81 issue 13, pp 1268–1274. doi:10.1021/j100528a010
- ^ a b c d e f Dario Braga, Lucia Maini, Fabrizia Grepioni (2002), Croconic Acid and Alkali Metal Croconate Salts: Some New Insights into an Old Story. Chemistry - A European Journal, Volume 8 Issue 8, Pages 1804–1812 doi:10.1002/1521-3765(20020415)8:8<1804::AID-CHEM1804>3.0.CO;2-C
- ^ J. D. Dunitz, P. Seiler, W. Czchtizky (2001), Angew. Chem. vol. 113, p. 1829; Angew. Chem. Int. Ed. vol 40, p. 1779
See also
- Rhodizonic acid
- Squaric acid
- Deltic acid
- Cyclopentanepentone (leuconic acid)
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