Croconic acid

Source: Wikipedia, the free encyclopedia.
Not to be confused with
Crocinic acid
.
Croconic acid
Skeletal formula
Ball-and-stick model
Space-filling model
Names
Preferred IUPAC name
4,5-Dihydroxycyclopent-4-ene-1,2,3-trione
Other names
Crocic acid
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.201.686 Edit this at Wikidata
UNII
  • InChI=1S/C5H2O5/c6-1-2(7)4(9)5(10)3(1)8/h6-7H checkY
    Key: RBSLJAJQOVYTRQ-UHFFFAOYSA-N checkY
  • O=C1C(O)=C(O)C(=O)C1=O
Properties
C5H2O5
Molar mass 142.07
Melting point > 300 °C (572 °F; 573 K) (decomposes)
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).

Croconic acid (also known as 4,5-dihydroxycyclopentenetrione, crocic acid or pentagonic acid) is a

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

anions, hydrogencroconate C5HO−5[1] and croconate C5O2−5 are also quite stable. The croconate ion, in particular, is aromatic[6] 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[7] 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.[7] Salts of barium, lead, silver, and others[specify] are also known.[1]

Croconic acid also forms

alkyl
group.

History

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".[7] The structure of ammonium croconate was determined by Baenziger et al. in 1964. The structure of K2C5O5·2H2O was determined by Dunitz in 2001.[8]

Structure

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 C5O5H2 held together by

dioxane
it has a large dipole moment of 9–10 
Curie point above 400 K (127 °C), 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.[9]

In the solid alkali metal salts, the croconate anions and the alkali cations form parallel columns.[7] In the mixed salt K3(HC5O5)(C5O5)·2H2O, which formally contains both one croconate dianion C5O2−5 and one hydrogencroconate monoanion (HC5O−5), the hydrogen is shared equally by two adjacent croconate units.[7]

Salts of the croconate anion and its derivatives are of interest in

π-stacking effects, where the delocalized electrons of two stacked croconate anions interact.[10]

Infrared and Raman assignments indicate that the equalization of the carbon–carbon bond lengths, thus the electronic delocalization, follows with an increase in counter-ion size for salts.[6] This result leads to a further interpretation that the degree of aromaticity is enhanced for salts as a function of the size of the counter-ion. The same study provided quantum mechanical DFT calculations for the optimized structures and vibrational spectra which were in agreement with experimental findings. The values for calculated theoretical indices of aromaticity also increased with counterion size.

The croconate anion forms

orthorhombic crystal structure, consisting of chains of alternating croconate and metal ions. Each croconate is bound to the preceding metal by one oxygen atom, and to the next metal through its two opposite oxygens, leaving two oxygens unbound. Each metal is bound to three croconate oxygens and to one water molecule.[11] Calcium also forms a compound with the same formula (yellow) but the structure appears to be different.[11]

Croconate dianion

The croconate anion also forms compounds with trivalent cations such as

sandwich-type bonds between the delocalized electrons and the metal (as are seen in ferrocene, for example),[11] but the anion can form metal complexes with a large variety of bonding patterns, involving from only one to all five of its oxygen atoms.[12][13][14]

See also

References

  1. ^
    doi:10.1246/bcsj.31.543
    .
  2. ^ Miller, W. A. (1868). Elements of Chemistry: Theoretical and Practical (4th ed.). Longmans.[page needed]
  3. ^ Turner, E. Elements of Chemistry.[page needed]
  4. ^
    doi:10.1021/j100588a009
    .
  5. doi:10.1021/j100528a010
    .
  6. ^
    doi:10.1016/j.molstruc.2006.01.035
    .
  7. ^
    PMID 11933108
    .
  8. PMID 11353510
    .
  9. ^
    S2CID 205219520
    .
  10. doi:10.1016/j.ica.2009.09.050
    .
  11. ^
    doi:10.1021/ja00900a013
    .
  12. doi:10.1016/j.ica.2008.12.002
    .
  13. doi:10.1016/j.poly.2006.09.100
    .
  14. ISSN 1528-7483
    .

External links
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