Squaric acid

Last updated
Squaric acid [1]
Structural formula (carbon atoms omitted) Squaric acid.svg
Structural formula (carbon atoms omitted)
Ball-and-stick-model Squaric-acid-3D-balls.png
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 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 220-761-4
PubChem CID
UNII
  • InChI=1S/C4H2O4/c5-1-2(6)4(8)3(1)7/h5-6H Yes check.svgY
    Key: PWEBUXCTKOWPCW-UHFFFAOYSA-N Yes check.svgY
  • 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
Appearancewhite crystalline powder
Melting point >300 °C (572 °F; 573 K)
Acidity (pKa)pKa1 = 1.5
pKa2 = 3.4
Hazards [2]
GHS labelling:
GHS-pictogram-acid.svg
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).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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]

Contents

The conjugate base of squaric acid is the hydrogensquarate anion HC4O4; 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.

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 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.png
Squaric acid dianion resonance forms
Squarate-anion-3D-balls.png
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 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]

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 deltate C3O2−3 and acetylenediolate C2O2−2 are obtainable by reductive coupling of carbon monoxide using organouranium complexes. [17] [18]

See also

Related Research Articles

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Tetraphenylphosphonium chloride is the chemical compound with the formula [(C6H5)4P]Cl, abbreviated Ph4PCl or PPh4Cl or [PPh4]Cl, where Ph stands for phenyl. Tetraphenylphosphonium and especially tetraphenylarsonium salts were formerly of interest in gravimetric analysis of perchlorate and related oxyanions. This colourless salt is used to generate lipophilic salts from inorganic and organometallic anions. Thus, [Ph4P]+ is useful as a phase-transfer catalyst, again because it allows inorganic anions to dissolve in organic solvents.

<span class="mw-page-title-main">Oxocarbon</span> Chemical compounds made of only carbon and oxygen

In chemistry, an oxocarbon or oxide of carbon is a chemical compound consisting only of carbon and oxygen. The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide. Many other stable or metastable oxides of carbon are known, but they are rarely encountered, such as carbon suboxide and mellitic anhydride.

<span class="mw-page-title-main">Cyclopentanepentone</span> Chemical compound

Cyclopentanepentone, also known as leuconic acid, is a hypothetical organic compound with formula C5O5, the fivefold ketone of cyclopentane. It would be an oxide of carbon (an oxocarbon), indeed a pentamer of carbon monoxide.

<span class="mw-page-title-main">Ethylene dione</span> Chemical compound

Ethylene dione or ethylenedione, also called dicarbon dioxide, Carbon peroxide, ethenedione, or ethene-1,2-dione, is a chemical compound with the formula C2O2 or O=C=C=O. It is an oxide of carbon, and can be described as the carbon-carbon covalent dimer of carbon monoxide. It can also be thought of as the dehydrated form of glyoxylic acid, or a ketone of ethenone H2C=C=O.

<span class="mw-page-title-main">Tetracarbon dioxide</span> Chemical compound

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<span class="mw-page-title-main">Deltic acid</span> Chemical compound

Deltic acid is a chemical substance with the chemical formula C3O(OH)2. It can be viewed as a ketone and double enol of cyclopropene. At room temperature, it is a stable white solid, soluble in diethyl ether, that decomposes between 140 °C and 180 °C, and reacts slowly with water.

<span class="mw-page-title-main">Acetylenediol</span> Chemical compound

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<span class="mw-page-title-main">Acetylenedicarboxylic acid</span> Chemical compound

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<span class="mw-page-title-main">Oxocarbon anion</span> Negatively-charged molecule made of carbon and oxygen

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x
On
y
for some integers x, y, and n.

<span class="mw-page-title-main">Dicarbonate</span> Chemical compound

A dicarbonate, also known as a pyrocarbonate, is a chemical containing the divalent −O−C(=O)−O−C(=O)−O− or −C2O5 functional group, which consists of two carbonate groups sharing an oxygen atom. These compounds can be viewed as derivatives of the hypothetical compound dicarbonic acid, HO−C(=O)−O−C(=O)−OH or H2C2O5. Two important organic compounds containing this group are dimethyl dicarbonate H3C−C2O5−CH3 and di-tert-butyl dicarbonate(H3C−)3C−C2O5−C(−CH3)3.

<span class="mw-page-title-main">Croconate violet</span>

Croconate violet or 1,3-bis(dicyanomethylene)croconate is a divalent anion with chemical formula C
11
N
4
O2−
3
or ((N≡C−)2C=)2(C5O3)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C
5
O2−
5
through the replacement of two oxygen atoms by dicyanomethylene groups =C(−C≡N)2. Its systematic name is 3,5-bis(dicyanomethylene)-1,2,4-trionate. The term croconate violet as a dye name specifically refers to the dipotassium salt K
2
C
11
N
4
O
3
.

<span class="mw-page-title-main">1,2-Bis(dicyanomethylene)squarate</span>

1,2-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C
10
N
4
O2−
2
or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C
4
O2−
4
through the replacement of two adjacent oxygen atoms by dicyanomethylene groups =C(−C≡N)2.

<span class="mw-page-title-main">1,3-Bis(dicyanomethylene)squarate</span>

1,3-Bis(dicyanomethylene)squarate is a divalent anion with chemical formula C
10
N
4
O2−
2
or ((N≡C−)2C=)2(C4O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion C
4
O2−
4
through the replacement of two opposite oxygen atoms by dicyanomethylene groups =C(−C≡N)2.

<span class="mw-page-title-main">Croconate blue</span>

Croconate blue or 1,2,3-tris(dicyanomethylene)croconate is a divalent anion with chemical formula C
14
N
6
O2−
2
or ((N≡C−)2C=)3(C5O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C
5
O2−
5
through the replacement of three oxygen atoms by dicyanomethylene groups =C(−C≡N)2. The term Croconate Blue as a dye name specifically refers to the dipotassium salt K
2
C
14
N
6
O
2
.

<span class="mw-page-title-main">2-(Dicyanomethylene)croconate</span> Ion

2-(Dicyanomethylene)croconate is a divalent anion with chemical formula C
8
N
2
O2−
4
or ((N≡C−)2C=)(C5O4)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C
5
O2−
5
through the replacement of one oxygen atom by a dicyanomethylene group =C(−C≡N)2.

<span class="mw-page-title-main">Pseudo-oxocarbon anion</span>

In chemistry, the term pseudo-oxocarbon anion is used to refer to a negative ion that is conceptually derived from an oxocarbon anion through replacement of one or more of the basic oxygen atoms by chemically similar elements or functional groups, such as sulfur (S), selenium (Se), or dicyanomethylene (=C(CN)2).

In chemistry, decarbonylation is a type of organic reaction that involves the loss of carbon monoxide (CO). It is often an undesirable reaction, since it represents a degradation. In the chemistry of metal carbonyls, decarbonylation describes a substitution process, whereby a CO ligand is replaced by another ligand.

<span class="mw-page-title-main">Metal salen complex</span> Coordination complex

A metal salen complex is a coordination compound between a metal cation and a ligand derived from N,N′-bis(salicylidene)ethylenediamine, commonly called salen. The classical example is salcomine, the complex with divalent cobalt Co2+, usually denoted as Co(salen). These complexes are widely investigated as catalysts and enzyme mimics.

References

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