Squaric acid

Squaric acid is a white crystalline powder.{{cite journal | last1= Lee |first1=K.-S. |last2=Kweon |first2=J. J. |last3=Oh |first3=I.-H. |last4=Lee |first4=C. E. | year = 2012 | title = Polymorphic phase transition and thermal stability in squaric acid ({{chem|H|2|C|4|O|4|}}) | journal = J. Phys. Chem. Solids | volume = 73 | issue = 7| pages = 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 pK_a1 = 1.5 for the first proton and pK_a2 = 3.4 for the second is attributable to resonance stabilization of the anion.{{cite journal | author1-link = Robert West (chemist)|first1=Robert|last1= West |first2=David L.|last2= Powell | year = 1963 | title = New Aromatic Anions. III. Molecular Orbital Calculations on Oxygenated Anions | journal = J. Am. Chem. Soc. | volume = 85 | issue = 17| pages = 2577–2579 | doi=10.1021/ja00900a010}} 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.

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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.{{cite web |title=Acidity Tables for Heteroatom Organic Acids and Carbon Acids|url=https://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/acidity2.htm}}

:{{chem2|C4O2(OH)2 → [C4O3(OH)]− + H+}}, pK_a1 = 1.5

:{{chem2|[C4O3(OH)]− → [C4O4](2−) + H+}}, pK_a2 = 3.5

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

:{{chem2|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.{{cite journal |title=Reaktionen von Quadratsäure und Quadratsäure-Derivaten|journal=Synthesis|year=1980|author=Arthur H. Schmidt|volume=1980|issue=12|page=961|doi=10.1055/s-1980-29291|s2cid=101871124 }}

The bis(methylether) is prepared by alkylation with trimethyl orthoformate.{{cite journal |doi=10.15227/orgsyn.076.0189|title=Dimethyl Squarate and ITS Conversion to 3-Ethenyl-4-Methoxycyclobutene-1,2-Dione and 2-Butyl-6-Ethenyl-5-Methoxy-1,4-Benzoquinone|journal=Organic Syntheses|year=1999|volume=76|page=189 |first1=Hui |last1=Liu|first2=Craig S. |last2=Tomooka|first3=Simon L.|last3=Xu|first4=Benjamin R.|last4=Yerxa|first5=Robert W.|last5=Sullivan|first6=Yifeng|last6=Xiong|first7=Harold W.|last7=Moore}}

Dibutyl squarate is used for the treatment of warts{{cite journal | doi = 10.1067/mjd.2000.103631 | title = Squaric acid immunotherapy for warts in children | year = 2000 | last1 = Silverberg | first1 = Nanette B. | last2 = Lim | first2 = Joseph K. | last3 = Paller | first3 = Amy S. | last4 = Mancini | first4 = Anthony J. | journal = Journal of the American Academy of Dermatology | volume = 42 | issue = 5 | pages = 803–808 | pmid = 10775858}} and for alopecia areata .{{cite journal | doi = 10.1016/j.autrev.2016.02.021 | title = Modified immunotherapy for alopecia areata | year = 2016 | last1 = Yoshimasu | first1 = Takashi | last2 = Furukawa | first2 = Fukumi | journal = Autoimmunity Reviews | volume = 15 | issue = 7 | pages = 664–667 | pmid = 26932732}}

Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.{{cn|date=January 2023}}

Squaramides are prepared by displacement of alkoxy or chloride groups from {{chem2|C4O2X2}} (X = OR, Cl).{{Cite journal | journal = Chem. Soc. Rev. | date = 2011 | volume = 40 | pages = 2330–2346 | doi = 10.1039/c0cs00200c | title=Squaramides: Physical Properties, Synthesis and Applications| last1 = Ian Storer | first1 = R. | last2 = Aciro | first2 = Caroline | last3 = Jones | first3 = Lyn H. | issue = 5 | pmid = 21399835}}

One or both of the oxygen (=O) groups in the squarate anion can be replaced by dicyanomethylene {{chem2|\dC(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 {{convert|10|K|C|0}} affords acetylenediol.{{cite journal | last1= Maier |first1=Günther |last2=Rohr |first2=Christine | year = 1995 | title = Ethynediol: Photochemical generation and matrix-spectroscopic identification. | journal = Liebigs Annalen | volume = 1996 | issue = 3| pages = 307–309 | doi = 10.1002/jlac.199619960303}}

Squarate dianion behaves similarly to oxalate, forming mono- and polynuclear complexes with hard metal ions.

Cobalt(II) squarate hydrate {{chem2|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).{{cite journal | first1= Kumagai|last1= Hitoshi |first2=Sobukawa |last2=Hideo |first3=Kurmoo |last3=Mohamedally | year = 2008 | title = Hydrothermal syntheses, structures and magnetic properties of coordination frameworks of divalent transition metals | journal = Journal of Materials Science | volume = 43 | issue = 7| pages = 2123–2130 | doi = 10.1007/s10853-007-2033-8 |bibcode= 2008JMatS..43.2123K |s2cid= 95205908}}

Cobalt(II) squarate dihydroxide {{chem2|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.

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

The same method yields iron(II) squarate dihydroxide {{chem2|Fe2(OH)2(C4O4)}} (light brown).

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.{{cite journal | title = Hydrolysis Reactions of Halogenated Cyclobutene Ethers: Synthesis of Diketocyclobutenediol |first1=J. D. |last1=Park |first2=S. |last2=Cohen |first3=J. R. |last3=Lacher |name-list-style=amp | journal = J. Am. Chem. Soc. |year = 1962 | volume = 84 | issue = 15 | pages = 2919–2922 | doi = 10.1021/ja00874a015}}

Although impractical, squarate and related anions such as deltate {{chem2|C3O3(2-)}} and acetylenediolate {{chem2|C2O2(2-)}} are obtainable by reductive coupling of carbon monoxide using organouranium complexes.{{cite journal | last1= Frey |first1=Alistair S. |last2=Cloke |first2=F. Geoffrey N. |last3=Hitchcock |first3=Peter B. | year = 2008 | title = 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 = Journal of the American Chemical Society | volume = 130 | issue = 42| pages = 13816–13817 | doi = 10.1021/ja8059792 |pmid=18817397}}{{cite journal | last1= Summerscales |first1=Owen T. |last2=Frey |first2=Alistair S. P. |last3=Cloke |first3=F. Geoffrey N. |last4=Hitchcock |first4=Peter B. | year = 2009 | title = Reductive disproportionation of carbon dioxide to carbonate and squarate products using a mixed-sandwich U(III) complex | journal = Chemical Communications | issue = 2| pages = 198–200 | doi = 10.1039/b815576c |pmid=19099067}}

• Acetylenediol, {{chem2|H2(CO)2}} or {{chem2|HO\sC\tC\sOH}}
• Deltic acid, {{chem2|H2(CO)3}}
• Croconic acid, {{chem2|H2(CO)5}}
• Rhodizonic acid, {{chem2|H2(CO)6}}
• Cyclopropenone, {{chem2|C3H2O}}
• Cyclobutene, {{chem2|C4H6}}
• Squaramide, {{chem2|C4O2(NH2)2}}, a nitrogen analog of squaric acid, where the OH groups of squaric acid are replaced by Amine
• Moniliformin, {{chem2|NaC4HO3}}, the sodium salt of semisquaric acid

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Category:Organic acids

Category:Diketones

Category:Enediols

Category:Cyclobutenes