| Squaric acid[1] | |
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3,4-Dihydroxycyclobut-3-ene-1,2-dione |
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Other names
Quadratic acid |
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| Identifiers | |
| CAS number | 2892-51-5 |
| PubChem | 17913 |
| ChemSpider | 16919 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C4H2O4 |
| Molar mass | 114.06 g/mol |
| Appearance | Gray powder |
| Melting point |
>300 °C |
| Hazards | |
| R-phrases | R36/37/38 R43 |
| S-phrases | S26 S36 |
| Flash point | 190 °C[2] |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula C4H2O4.
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.
Contents |
Squaric acid is a white crystalline powder with a melting point of 293 °C. 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 C4S2−
4 is anti-aromatic.[4]
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 reasplaced 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]
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 C3O2−
3 and acetylenediolate C2O2−
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]
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]