Squaric acid or quadratic acid (so named because its four carbon atoms approximately form a square) is a diprotic

organic acid An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...

with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

. The

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid gives a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as it loses a hydrogen ion in the reve ...

of squaric acid is the hydrogensquarate anion ; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion . This is one of the oxocarbon anions, which consist only of carbon and oxygen. Squaric acid is a reagent for

chemical synthesis Chemical synthesis (chemical combination) is the artificial execution of chemical reactions to obtain one or several products. This occurs by physical and chemical manipulations usually involving one or more reactions. In modern laboratory uses ...

, used for instance to make

photosensitive Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity. ...

squaraine dyes and inhibitors of

protein tyrosine phosphatase Protein tyrosine phosphatases (EC 3.1.3.48, systematic name protein-tyrosine-phosphate phosphohydrolase) are a group of enzymes that remove phosphate groups from phosphorylated tyrosine residues on proteins: : proteintyrosine phosphate + H2O = ...

Chemical properties

Squaric acid is a white crystalline powder. 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 An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the ...

with p''K''_a1 = 1.5 for the first proton and p''K''_a2 = 3.4 for the second is attributable to

resonance stabilization In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

of the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

. 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 length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...

s 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. :, p''K''_a1 = 1.5 :, p''K''_a2 = 3.5 The OH groups are labile in squaric acid. It forms a dichloride with

thionyl chloride Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately Volatility (chemistry), volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a Halogenation, chlorinating reagen ...

: : The chlorides are good leaving groups, reminiscent of

acid chloride In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example o ...

s. They are displaced by diverse nucleophiles. In this way dithiosquarate can be prepared. The bis(methylether) is prepared by alkylation with trimethyl orthoformate. Dibutyl squarate is used for the treatment of warts and for alopecia areata . Diethyl squarate has been used as an intermediate in the synthesis of perzinfotel.

Squaramide Squaramide is the organic compound with the formula O2C4(NH2)2. Not an amide in the usual sense, it is a derivative of squaric acid wherein the two OH groups are replaced by NH2 groups. Squaramides refer to a large class of derivatives wherein s ...

s are prepared by displacement of alkoxy or chloride groups from (X = OR, Cl). One or both of the oxygen (=O) groups in the squarate anion can be replaced by dicyanomethylene . 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 Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

of squaric acid in a solid argon matrix at affords acetylenediol.

Coordination complexes

Squarate dianion behaves similarly to

oxalate Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...

, forming mono- and polynuclear complexes with hard metal ions. Cobalt(II) squarate hydrate (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 Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...

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). Cobalt(II) squarate dihydroxide (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 Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...

; 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. Infrared, electronic and Q-Band EPR spectra as well as magnetic susceptibilities are reported. The same method yields iron(II) squarate dihydroxide (light brown).

Synthesis

The original synthesis started with the ethanolysis of hexafluorocyclobutene to give 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene.

Hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

gives the squaric acid. Although impractical, squarate and related anions such as deltate and acetylenediolate are obtainable by reductive coupling of carbon monoxide using organouranium complexes.

References

{{Authority control Organic acids Diketones Enediols Cyclobutenes

Chemical properties

Derivatives

Coordination complexes

Synthesis

See also

References