Squaric acid
Encyclopedia
Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound
with chemical formula
424.
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 anion
s, which consist only of carbon and oxygen.
Squaric acid is a reagent for chemical synthesis
, used for instance to make photosensitive squaraine dye
s and inhibitors of protein tyrosine phosphatase
s.
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. 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 length
s 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 is anti-aromatic.
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).
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.
The same method yields iron(II) squarate dihydroxide Fe2(OH)2(C4O4) (light brown).
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 anion
s. 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.
to perfluorocyclobutene. This compound was converted to 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene with ethanol
. Hydrolysis
gives the squaric acid.
Squarate and related anions such deltate
and acetylenediolate have been obtained from carbon monoxide under mild conditions by reductive coupling of CO ligands in organouranium complexes
. A similar route recently afforded carbonate anions (in the form of uranium(IV) carbonate) from carbon dioxide
CO2.
or alopecia totalis
/universalis (autoimmune hair loss) through topical immunotherapy
involving the production of an allergic rash.
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
with chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
424.
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 anion
Oxocarbon anion
In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula CxOyn− for some integers x, y, and n....
s, which consist only of carbon and oxygen.
Squaric acid is a reagent for chemical synthesis
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
, used for instance to make photosensitive squaraine dye
Squaraine dye
Squaraine dyes are a class of organic dyes showing intense fluorescence, typically in the red and near infrared region . They are characterized by their unique aromatic four membered ring system derived from squaric acid...
s and inhibitors of protein tyrosine phosphatase
Protein tyrosine phosphatase
Protein tyrosine phosphatases are a group of enzymes that remove phosphate groups from phosphorylated tyrosine residues on proteins. Protein tyrosine phosphorylation is a common post-translational modification that can create novel recognition motifs for protein interactions and cellular...
s.
Chemical properties
Squaric acid is a white crystalline powder with a melting pointMelting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
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
PKA
PKA, pKa, or other similar variations may stand for:* pKa, the symbol for the acid dissociation constant at logarithmic scale* Protein kinase A, a class of cAMP-dependent enzymes* Pi Kappa Alpha, the North-American social fraternity...
= 1.5 for the first proton and pKa = 3.4 for the second is attributable to resonance stabilization
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
of the anion. 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 length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
s identical.
Another, quantum mechanical
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
, way of describing the dianion is to assume that the π electrons
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
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
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
is left in the ring of carbon atoms. In this way the ring fits Hückel's rule
Hückel's rule
In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931...
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 is anti-aromatic.
Derivatives
Cobalt(II) squarate hydrate Co(C4O4)(H2O)2 (yellow, cubic) can be prepared by autoclavingAutoclave
An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, although a precursor known as the...
cobalt(II) hydroxide
Cobalt(II) hydroxide
Cobalt hydroxide or cobaltous hydroxide is the chemical compound composed of cobalt and the hydroxide ion with the formula Co2. It occurs in two forms, either as a rose-red powder, which is the more stable of the two forms, or as bluish-green powder...
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).
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.
The same method yields iron(II) squarate dihydroxide Fe2(OH)2(C4O4) (light brown).
One or both of the oxygen (=O) groups in the squarate anion can be reasplaced by other chalcogenides such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or other divalent groups, such as dicyanomethylene =C(CN)2. The resulting anions, such as 1,2-bis(dicyanomethylene)squarate
1,2-Bis(dicyanomethylene)squarate
1,2-Bissquarate is a divalent anion with chemical formula or 22−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion through the replacement of two adjacent oxygen atoms by dicyanomethylene groups =C2.The anion can be obtained...
and 1,3-bis(dicyanomethylene)squarate
1,3-Bis(dicyanomethylene)squarate
1,3-Bissquarate is a divalent anion with chemical formula or 22−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the squarate oxocarbon anion through the replacement of two opposite oxygen atoms by dicyanomethylene groups =C2.The anion can be obtained...
, retain the aromatic character of squarate and have been called pseudo-oxocarbon anion
Pseudo-oxocarbon anion
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 , selenium , or dicyanomethylene...
s. 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.
Syntheses
The original synthesis started from reaction of 1-chloro-1,2,2-trifluoroethylene with zincZinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
to perfluorocyclobutene. This compound was converted to 1,2-diethoxy-3,3,4,4-tetrafluoro-1-cyclobutene with ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
. Hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
gives the squaric acid.
Squarate and related anions such deltate
Deltic acid
Deltic acid or dihydroxycyclopropenone is a chemical substance with the chemical formula C3O2. It can be viewed as a ketone and double alcohol of cyclopropene...
and acetylenediolate have been obtained from carbon monoxide under mild conditions by reductive coupling of CO ligands in organouranium complexes
Organouranium chemistry
Organouranium chemistry is the science exploring the properties, structure and reactivity of organouranium compounds, which are organometallic compounds containing a carbon to uranium chemical bond...
. A similar route recently afforded carbonate anions (in the form of uranium(IV) carbonate) from carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
CO2.
Medical uses
Medically, squaric acid dibutylester is used for the treatment of warts. Squaric acid dibutylester is also used treating alopecia areataAlopecia areata
Alopecia areata is a medical condition in which hair is lost from some or all areas of the body, usually from the scalp. Because it causes bald spots on the scalp, especially in the first stages, it is sometimes called spot baldness. In 1–2% of cases, the condition can spread to the entire scalp ...
or alopecia totalis
Alopecia totalis
Alopecia totalis is the loss of all head hair. Its causes are unclear, but it is currently believed to be an autoimmune disorder. Stress is sometimes thought to be a contributor in alopecia areata...
/universalis (autoimmune hair loss) through topical immunotherapy
Immunotherapy
Immunotherapy is a medical term defined as the "treatment of disease by inducing, enhancing, or suppressing an immune response". Immunotherapies designed to elicit or amplify an immune response are classified as activation immunotherapies. While immunotherapies that reduce or suppress are...
involving the production of an allergic rash.
See also
- CyclobuteneCyclobuteneCyclobutene is a cycloalkene. It is used in the chemical industry as a monomer for synthesis of some polymers and for a range of chemical syntheses.- External links :*...
, - Deltic acidDeltic acidDeltic acid or dihydroxycyclopropenone is a chemical substance with the chemical formula C3O2. It can be viewed as a ketone and double alcohol of cyclopropene...
, - Croconic acidCroconic acidCroconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula C5H2O5 or 32. It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms...
, - Rhodizonic acidRhodizonic acidRhodizonic acid is a chemical compound with formula C6H2O6 or 42. It can be seen as a two-fold alcohol and four-fold ketone of cyclohexene, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone....
,