Jump to content

Squaric acid

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Tassedethe (talk | contribs) at 22:38, 18 August 2012 (WPCleaner v1.15 - Repaired 1 link to disambiguation page - (You can help) - Robert West). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Squaric acid[1]
Structural formula (carbon atoms omitted)
Structural formula (carbon atoms omitted)
Ball-and-stick-model
Ball-and-stick-model
Names
IUPAC name
3,4-Dihydroxycyclobut-3-ene-1,2-dione
Other names
Quadratic acid
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.018.875 Edit this at Wikidata
  • InChI=1S/C4H2O4/c5-1-2(6)4(8)3(1)7/h5-6H checkY
    Key: PWEBUXCTKOWPCW-UHFFFAOYSA-N checkY
  • InChI=1/C4H2O4/c5-1-2(6)4(8)3(1)7/h5-6H
    Key: PWEBUXCTKOWPCW-UHFFFAOYAC
  • c1(c(c(=O)c1=O)O)O
Properties
C4H2O4
Molar mass 114.06 g/mol
Appearance Gray powder
Melting point >300 °C
Hazards
Flash point 190 °C[2]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula Template:Carbon4Template:Hydrogen2Template:Oxygen4.

The conjugate base of squaric acid is the hydrogensquarate anion C4HO4−; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O42−. This is one of the oxocarbon anions, which consist only of carbon and oxygen.

Squaric acid is a reagent for chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases.

Chemical properties

Squaric acid is a white crystalline powder with a thermal decomposition point of 245 °C at ambient pressure [ K.-S. Lee, J. J. Kweon, I.-H. Oh, C. E. Lee (2012). "Polymorphic phase transition and thermal stability in squaric acid (H
2C
4O
4)". J. Phys. Chem. Solids 73 (7): 890–895. doi:10.1016/j.jpcs.2012.02.013 ]. The onset of thermal decomposition depends on the different thermodynamic conditions such as heating rates.

The structure of squaric acid is not a perfect square, as the carbon–carbon bond lengths are not quite equal. The high acidity with pKa = 1.5 for the first proton and pKa = 3.4 for the second is attributable to resonance stabilization of the anion.[3] Because the negative charges are equally distributed between each oxygen atom, the dianion of squaric acid is completely symmetrical (unlike squaric acid itself) with all C-C and C-O bond lengths identical.

Another, quantum mechanical, way of describing the dianion is to assume that the π electrons of the two double-bonded oxygen atoms are shifted to the latter, so that all four oxygens become single-bonded -O− groups and a double positive electric charge is left in the ring of carbon atoms. In this way the ring fits Hückel's rule for aromaticity (2 π-electrons = 4n + 2 with n = 0). The total symmetry of the dianion is a consequence of charge distribution and aromaticity.

On the other hand, theoretical calculations indicate that the analogous tetrathiosquarate anion C
4S2−
4 is anti-aromatic.[4]

Derivatives

Cobalt(II) squarate hydrate Co(C4O4)(H2O)2 (yellow, cubic) can be prepared by autoclaving cobalt(II) hydroxide and squaric acid in water at 200 °C. The water is bound to the cobalt atom, and the crystal structure consists of a cubic arrangement of hollow cells, whose walls are either six squarate anions (leaving a 7Å wide void) or several water molecules (leaving a 5 Å void).[5]

Cobalt(II) squarate dihydroxide Co3(OH)2(C4O4)2 3H2O (brown) is obtained together with the previous compound. It has a columnar structure including channels filled with water molecules; these can be removed and replaced without destroying the crystal structure. The chains are ferromagnetic; they are coupled antiferromagnetically in the hydrated form, ferromagnetically in the anhydrous form.[5]

The same method yields iron(II) squarate dihydroxide Fe2(OH)2(C4O4) (light brown).[5]

One or both of the oxygen (=O) groups in the squarate anion can be replaced by other chalcogenides such as sulfur or other divalent groups, such as dicyanomethylene =C(CN)2. The resulting anions, such as 1,2-bis(dicyanomethylene)squarate and 1,3-bis(dicyanomethylene)squarate, retain the aromatic character of squarate and have been called pseudo-oxocarbon anions. There have been theoretical investigations of the analogous compound obtained by substituting amino groups (-NH2) for the hydroxyl (OH) groups to yield 1,2-diamino-3-cyclobutenedione, and of a compound consisting of two squarate rings bridged by (-NH-) bonds to form bis(3-cyclobutene-1,2-dione)piperazine.[6]

Syntheses

The original synthesis started from reaction of 1-chloro-1,2,2-trifluoroethylene with zinc to perfluorocyclobutene. This compound was converted to 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene with ethanol. Hydrolysis gives the squaric acid. [7]

Squarate and related anions such deltate C
3O2−
3 and acetylenediolate C
2O2−
2 have been obtained from carbon monoxide under mild conditions by reductive coupling of CO ligands in organouranium complexes.[8] A similar route recently afforded carbonate anions (in the form of uranium(IV) carbonate) from carbon dioxide CO2.[9]

Medical uses

Medically, squaric acid dibutylester is used for the treatment of warts.[10] Squaric acid dibutylester is also used treating alopecia areata or alopecia totalis/universalis (autoimmune hair loss) through topical immunotherapy involving the production of an allergic rash.[11]

See also

References

  1. ^ 3,4-Dihydroxy-3-cyclobutene-1,2-dione. Sigma-Aldrich
  2. ^ 3,4-Dihydroxy-3-cyclobutene-1,2-dione, 98+%. Alfa Aesar
  3. ^ Robert West and David L. Powell (1963), New Aromatic Anions. III. Molecular Orbital Calculations on Oxygenated Anions J. Am. Chem. Soc. volume 85 issue 17, pages 2577–2579.
  4. ^ Reza Ghiasi and Majid Monajjemi (2007), Theoretical study of interaction of alkaline earth metal with C
    4    O2−
    4     and C
    4    S2−
    4    : structure, electronic properties and aromaticity    . Journal of Sulfur Chemistry, Volume 28, Issue 6, pages 537–546 doi:10.1080/17415990701561263
  5. ^ a b c Hitoshi Kumagai, Hideo Sobukawa, and Mohamedally Kurmoo (2008), Hydrothermal syntheses, structures and magnetic properties of coordination frameworks of divalent transition metals. Journal of Materials Science volume 43, pages 2123–2130. doi:10.1007/s10853-007-2033-8
  6. ^ Zhao-Ming Xue, Jian-Jun Cheng, and Chun-Hua Chen (2006), Theoretical study of the gas-phase acidity and aromaticity of a novel derivative of nitrogen squaric acid . Journal of Molecular Structure: THEOCHEM, Volume 763, Issues 1–3, pages 181–186 doi:10.1016/j.theochem.2006.01.026
  7. ^ J. D. Park, S. Cohen, and J. R. Lacher (1962). "Hydrolysis Reactions of Halogenated Cyclobutene Ethers: Synthesis of Diketocyclobutenediol". J. Am. Chem. Soc. 84 (15): 2919–2922. doi:10.1021/ja00874a015.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Alistair S. Frey, F. Geoffrey N. Cloke, Peter B. Hitchcock (2008), Mechanistic Studies on the Reductive Cyclooligomerisation of CO by U(III) Mixed Sandwich Complexes; the Molecular Structure of [(U(η-C8H6{Si'Pr3-1,4}2)(η-Cp*)]2(μ-η1:η1-C2O2) Journal of the American Chemical Society, volume 130, issue 42, pages 13816–13817. doi:10.1021/ja8059792
  9. ^ Owen T. Summerscales, Alistair S. P. Frey, F. Geoffrey N. Cloke, and Peter B. Hitchcock (2009), Reductive disproportionation of carbon dioxide to carbonate and squarate products using a mixed-sandwich U(III) complex. Chemical Communications, pages 198–200 doi:10.1039/b815576c
  10. ^ Warts. Wilmingtondermatologycenter.com. Retrieved on 2011-10-23.
  11. ^ A. M. Holzer, L. L. Kaplan, W. R. Levis (2006). "Haptens as drugs: contact allergens are powerful topical immunomodulators". J. Drugs. Dermatol. 5 (5): 410–416. PMID 16703776.{{cite journal}}: CS1 maint: multiple names: authors list (link)
Retrieved from "http://en.wiki.x.io/w/index.php?title=Squaric_acid&oldid=508045871"