What is "croconic acid"

Croconic \Cro*con"ic\ (kr?-k?n"?k), a. [Gr. ???? saffron.]

1. Of, pertaining to, or resembling saffron; having the color of saffron; as, croconic acid.

2. Pertaining to, or derived from, croconic acid.

   Croconic acid (Chem.), a yellow crystalline substance, C5O3(OH)2, obtained from potassium carboxide, rhodizonic acid, and various phenol and quinone derivatives of benzene, and forming yellow or orange colored salts.

n. (context organic compound English) A derivative of cyclopentene having two hydroxyl groups and three carbonyl groups; its anion (croconate) is aromatic

Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula CHO or (C=O)(COH). 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.

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 CHO and croconate CO 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

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.