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|Section2= }} Croconic acid or 4,5-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,〔 soluble in water and ethanol〔 William Allen Miller (1868), ''Elements of chemistry: theoretical and practical'', 4th edition. 884 pages. Longmans 〕 and forms yellow crystals that decompose at 212 °C.〔Edward Turner, ''Elements of Chemistry''〕 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).〔 Lowell M. Schwartz, Robert I. Gelb, and Janet O. Yardley (1975), ''Aqueous Dissociation of Croconic Acid''. J. Phys. Chem., volume 79 issue 21, pp 2246–2251. 〕〔 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. 〕 The resulting anions, hydrogencroconate C5HO5− 〔 and croconate C5O52− are also quite stable. The croconate ion, in particular, is aromatic〔 Georgopoulos SL, Diniz R, Yoshida MI, Speziali NL, Dos Santos HF, Junqueira GMA, de Oliveira LFC (2006) "Vibrational spectroscopy and aromaticity investigation of squarate salts: A theoretical and experimental approach", JOURNAL OF MOLECULAR STRUCTURE Volume: 794 Issue: 1-3 Pages: 63-70 〕 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〔 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 〕 but the orange potassium salt can be dehydrated to form a monohydrate. 〔 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 . 〕〔 The croconates of ammonium, rubidium and caesium crystallize in the anhydrous form.〔 Salts of barium, lead, silver, etc. are also known.〔 Croconic acid also forms ethers such as dimethyl croconate where the hydrogen atom of the hydroxyl group is substituted with an alkyl group. ==History== Croconic acid and potassium croconate dihydrate were discovered by Leopold Gmelin in 1825, who named the compounds from Greek ''κρόκος'' meaning "saffron" or "egg yolk".〔 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 〔J. D. Dunitz, P. Seiler, W. Czchtizky (2001), Angew. Chem. vol. 113, p. 1829; Angew. Chem. Int. Ed. vol 40, p. 1779 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Croconic acid」の詳細全文を読む スポンサード リンク
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