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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]
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| Names | |||
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| Preferred IUPAC name
3,4-Dihydroxycyclobut-3-ene-1,2-dione | |||
| Other names
Quadratic acid Cyclobutenedioic acid | |||
| Identifiers | |||
3D model (JSmol) |
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| ChemSpider | |||
| ECHA InfoCard | 100.018.875 | ||
| EC Number |
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PubChem CID |
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| UNII | |||
CompTox Dashboard (EPA) |
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| 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: | |||
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| 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).
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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 chemical synthesis, used for instance to make photosensitive squaraine dyes and inhibitors of protein tyrosine phosphatases.
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 acidity with pKa1 = 1.5 for the first proton and pKa2 = 3.4 for the second is attributable to resonance stabilization of the 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
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 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.
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]
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.[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]
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 deltate C3O2−3 and acetylenediolate C2O2−2 are obtainable by reductive coupling of carbon monoxide using organouranium complexes.[17][18]
- Acetylenediol, H2(CO)2 or HO−C≡C−OH
- Deltic acid, H2(CO)3
- Croconic acid, H2(CO)5
- Rhodizonic acid, H2(CO)6
- Cyclopropenone, C3H2O
- Cyclobutene, C4H6
- Squaramide, C4O2(NH2)2, a nitrogen analog of squaric acid, where the OH groups of squaric acid are replaced by NH2 groups
- Moniliformin, NaC4HO3, the sodium salt of semisquaric acid
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