Squaric acid

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Squaric acid[1]
Structural formula (carbon atoms omitted)
Structural formula (carbon atoms omitted)
Ball-and-stick-model
Ball-and-stick-model
Names
Preferred IUPAC name
3,4-Dihydroxycyclobut-3-ene-1,2-dione
Other names
Quadratic acid
Cyclobutenedioic acid
Identifiers
3D model (
JSmol
)
ChemSpider
ECHA InfoCard
 100.018.875 Edit this at Wikidata
EC Number
  • 220-761-4
UNII
  • 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 114.056 g·mol−1
Appearance white crystalline powder
Melting point > 300 °C (572 °F; 573 K)
Acidity (pKa) pKa1 = 1.5
pKa2 = 3.4
Hazards[2]
GHS labelling:
GHS05: Corrosive
Danger
H314
P260, P280, P301+P330+P331, P303+P361+P353, P304+P340+P310, P305+P351+P338
Flash point 190 °C (374 °F; 463 K)[3]
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 a

diprotic organic acid with the chemical formula C4O2(OH)2.[4]

The

conjugate base of squaric acid is the hydrogensquarate anion HC4O−4; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O2−4. This is one of the oxocarbon anions
, which consist only of carbon and oxygen.

Squaric acid is a reagent for

photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases
.

Chemical properties

Squaric acid is a white crystalline powder.[5] 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

anion.[6] 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 bond lengths
identical and all C−O bond lengths identical.

Squaric acid dianion resonance forms
Ball-and-stick model of the squarate ion

Derivatives

Many of the reactions of squaric acid involve the OH groups. The molecule behaves similarly to a strong dicarboxylic acid. It is stronger acid than typical carboxylic acids.[7]

C4O2(OH)2 → [C4O3(OH)]− + H+,
pKa1
= 1.5
[C4O3(OH)]− → [C4O4]2− + H+, pKa2 = 3.5

The OH groups are labile in squaric acid. It forms a dichloride with thionyl chloride:

C4O2(OH)2 + 2 SOCl2 → C4O2Cl2 + 2 HCl + 2 SO2

The chlorides are good leaving groups, reminiscent of

acid chlorides. They are displaced by diverse nucleophiles. In this way dithiosquarate can be prepared.[8]

The bis(methylether) is prepared by alkylation with trimethyl orthoformate.[9]

Dibutyl squarate is used for the treatment of warts[10] and for alopecia areata .[11]

Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.[citation needed]

Squaramides are prepared by displacement of alkoxy or chloride groups from C4O2X2 (X = OR, Cl).[8][12]

One or both of the oxygen (=O) groups in the squarate anion can be replaced by

1,3-bis(dicyanomethylene)squarate, retain the aromatic character of squarate and have been called pseudo-oxocarbon anions
.

Photolysis of squaric acid in a solid argon matrix at 10 K (−263 °C) affords acetylenediol.[13]

Coordination complexes

Squarate dianion behaves similarly to oxalate, forming mono- and polynuclear complexes with hard metal ions. Cobalt(II) squarate hydrate Co(C4O4)·2H2O (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 7 Å wide void) or several water molecules (leaving a 5 Å void).[14]

ferromagnetic; they are coupled antiferromagnetically in the hydrated form, ferromagnetically in the anhydrous form.[14]

Copper(II) squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized.[15] Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported.

The same method yields iron(II) squarate dihydroxide Fe2(OH)2(C4O4) (light brown).[14]

Synthesis

The original synthesis started with the ethanolysis of perfluorocyclobutene to give 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene. Hydrolysis gives the squaric acid.[16][4]

Although impractical, squarate and related anions such as

acetylenediolate C2O2−2 are obtainable by reductive coupling of carbon monoxide using organouranium complexes.[17][18]

See also

References

  1. ^ 3,4-Dihydroxy-3-cyclobutene-1,2-dione. Sigma-Aldrich
  2. ^ "SICHERHEITSDATENBLATT". 21 March 2021.
  3. ^ 3,4-Dihydroxy-3-cyclobutene-1,2-dione, 98+%. Alfa Aesar
  4. ^
    ISBN 9780127445809
    .
  5. doi:10.1016/j.jpcs.2012.02.013
    .
  6. doi:10.1021/ja00900a010
    .
  7. ^ "Acidity Tables for Heteroatom Organic Acids and Carbon Acids".
  8. ^
    S2CID 101871124
    .
  9. doi:10.15227/orgsyn.076.0189
    .
  10. PMID 10775858
    .
  11. PMID 26932732
    .
  12. PMID 21399835
    .
  13. doi:10.1002/jlac.199619960303
    .
  14. ^
    S2CID 95205908
    .
  15. ^ Reinprecht, J. T.; Miller, J. G.; Vogel, G. C.; et al. (1979). "Synthesis and Characterization of Copper(II) Squarate Complexes". Inorg. Chem., 19, 927-931
  16. doi:10.1021/ja00874a015
    .
  17. PMID 18817397
    .
  18. PMID 19099067
    .
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