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Concept
Croconic acid
Summary
Croconic acid (also known as 4,5-dihydroxycyclopentenetrione, crocic acid or pentagonic acid) is a chemical compound with formula or . 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 and forms yellow crystals that decompose at 212 °C.
The compound is acidic and loses the protons from the hydroxyl groups (pKa1 = 0.80 and pKa2 = 2.24 at 25 °C). The resulting anions, hydrogencroconate and croconate 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 (with two electrons, Hückel's rule means this is an aromatic configuration). The lithium, sodium and potassium croconates crystallize from water as dihydrates but the orange potassium salt can be dehydrated to form a monohydrate. The croconates of ammonium, rubidium and caesium crystallize in the anhydrous form. Salts of barium, lead, silver, and others 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.
Croconic acid and potassium croconate dihydrate were discovered by Leopold Gmelin in 1825, who named the compounds from Greek
κρόκος meaning "crocus" or "egg yolk". The structure of ammonium croconate was determined by Baenziger et al. in 1964. The structure of was determined by Dunitz in 2001.
In the solid state, croconic acid has a peculiar structure consisting of pleated strips, each "page" of the strip being a planar ring of 4 molecules of held together by hydrogen bonds. In dioxane it has a large dipole moment of
9–10 D, while the free molecule is estimated to have a dipole of 7–7.5 D. The solid is ferroelectric with a Curie point above , indeed the organic crystal with the highest spontaneous polarization (about 20 μC/cm2). This is due to proton transfer between adjacent molecules in each pleated sheet, rather than molecular rotation.
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