Squaric acid
Squaric acid or quadratic acid (so named because its four carbon atoms approximately form a square) is a diprotic organic acid with the chemical formula Template:Chem2.[1]
The conjugate base of squaric acid is the hydrogensquarate anion Template:Chem2; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion Template:Chem2. 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.[2] 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.[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 bond lengths identical and all C−O bond lengths identical.
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.[4]
- Template:Chem2, pKa1 = 1.5
- Template:Chem2, pKa2 = 3.5
The OH groups are labile in squaric acid. It forms a dichloride with thionyl chloride:
The chlorides are good leaving groups, reminiscent of acid chlorides. They are displaced by diverse nucleophiles. In this way dithiosquarate can be prepared.[5]
The bis(methylether) is prepared by alkylation with trimethyl orthoformate.[6]
Dibutyl squarate is used for the treatment of warts[7] and for alopecia areata .[8]
Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.Script error: No such module "Unsubst".
Squaramides are prepared by displacement of alkoxy or chloride groups from Template:Chem2 (X = OR, Cl).[5][9]
One or both of the oxygen (=O) groups in the squarate anion can be replaced by dicyanomethylene Template:Chem2. 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 Template:Convert affords acetylenediol.[10]
Coordination complexes
Squarate dianion behaves similarly to oxalate, forming mono- and polynuclear complexes with hard metal ions. Cobalt(II) squarate hydrate Template:Chem2 (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).[11]
Cobalt(II) squarate dihydroxide Template:Chem2 (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.[11]
Copper(II) squarate monomeric and dimeric mixed-ligand complexes were synthesized and characterized.[12] Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported.
The same method yields iron(II) squarate dihydroxide Template:Chem2 (light brown).[11]
Synthesis
The original synthesis started with the ethanolysis of hexafluorocyclobutene to give 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene. Hydrolysis gives the squaric acid.[13][1]
Although impractical, squarate and related anions such as deltate Template:Chem2 and acetylenediolate Template:Chem2 are obtainable by reductive coupling of carbon monoxide using organouranium complexes.[14][15]
See also
- Acetylenediol, Template:Chem2 or Template:Chem2
- Deltic acid, Template:Chem2
- Croconic acid, Template:Chem2
- Rhodizonic acid, Template:Chem2
- Cyclopropenone, Template:Chem2
- Cyclobutene, Template:Chem2
- Squaramide, Template:Chem2, a nitrogen analog of squaric acid, where the OH groups of squaric acid are replaced by [[Amine|Template:Chem2 groups]]
- Moniliformin, Template:Chem2, the sodium salt of semisquaric acid
References
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- ↑ a b c Script error: No such module "Citation/CS1".
- ↑ 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
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