Croconic acid
Encyclopedia
Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound
with formula C5H2O5 or (C=O)3(COH)2. It has a cyclopentene
backbone with two hydroxyl
groups adjacent to the double bond and three ketone
groups on the remaining carbon
atoms. It is sensitive to light, soluble in water and ethanol
and forms yellow crystals that decompose at 212°C.
The compound is acidic and loses the hydrogen cations
H+ from the hydroxyls (pK1 = 0.80 ± 0.08 and pK2 = 2.24 ± 0.01 at 25°C). The resulting anions, hydrogencroconate C5HO5− and croconate C5O52− are also quite stable. The croconate ion, in particular, is aromatic
and symmetric, as the double bond and the negative charges become delocalized over the five CO units. The lithium
, sodium
and potassium
croconates crystallize from water as dihydrates
but the orange potassium salt can be dehydrated to form a monohydrate
.
The croconates of ammonium
, rubidium
and caesium
crystallize in the anhydrous form. Salts of barium
, lead
, silver
, etc. are also known.
Croconic acid also forms esters such as dimethyl croconate.
in 1825, who named the compounds from Greek
κρόκος meaning "saffron" or "egg yolk". The structure of ammonium croconate was determined by Baenziger et al. in 1964. The structure of K2C5O5 . 2H2O was determined by J. D. Dunitz in 2001
s. In dioxane it has a large dipole moment of
9 to 10 D
, while the free molecule is estimated to have a dipole of 7 to 7.5 D. The solid is ferroelectric with a Curie point
above 400 K, indeed the organic crystal with the highest spontaneous polarization (about 20 μC cm−2). This is due to proton transfer between adjacent molecules in each pleated sheet, rather than molecular rotation.
In the solid alkali metal
salts, the croconate anions and the alkali cations form parallel columns. In the mixed salt K3(HC5O5)(C5O5)·2H2O, which formally contains both the croconate dianion and the hydrogencroconate monoanion , the hydrogen is shared equally by two adjacent croconate units.
Salts of the croconate anion and its derivatives are of interest in supramolecular chemistry
research because of their potential for π-stacking effects, where the delocalized electrons of two stacked croconate anions interact.
The croconate anion forms hydrate
d crystalline coordination compounds with divalent cations of transition metals, with general formula M(C5O5)·3H2O; where M stands for copper
(yielding a brown solid), iron
(dark purple), zinc
(yellow), nickel
(green), manganese
(dark green), or cobalt
(purple). These complexes all have the same orthorombic crystal structure, consisting of chains of alternating croconate and metal ions. Each croconate is bound to the preceding metal by one oxygen
atom, and to the next metal through its two opposite oxygens, leaving two oxygens unbound. Each metal is bound to three croconate oxygens and to one water molecule. Calcium
also forms a compound with the same formula (yellow) but the structure appears to be different.
The croconate anion also forms compounds with trivalent cations such as aluminum (yellow), chromium
(brown), and iron (purple). These compounds also include hydroxyl groups as well as hydration water and have a more complicated crystal strcuture. No indication was found of sandwich-type bonds between the delocalized electrons and the metal (as are seen in ferrocene
, for example), but the anion can form metal complexes with a large variety of bonding patterns, involving from only one to all five of its oxygen atoms.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with formula C5H2O5 or (C=O)3(COH)2. It has a cyclopentene
Cyclopentene
Cyclopentene is a chemical compound with the formula 58. It is a colorless liquid with a petrol-like odor. It is one of the cycloalkenes.Cyclopentene is produced industrially in large amounts...
backbone with two hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
groups adjacent to the double bond and three ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
groups on the remaining carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms. It is sensitive to light, soluble in water and 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...
and forms yellow crystals that decompose at 212°C.
The compound is acidic and loses the hydrogen cations
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
H+ from the hydroxyls (pK1 = 0.80 ± 0.08 and pK2 = 2.24 ± 0.01 at 25°C). The resulting anions, hydrogencroconate C5HO5− and croconate C5O52− are also quite stable. The croconate ion, in particular, is aromatic
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
and symmetric, as the double bond and the negative charges become delocalized over the five CO units. The lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
, sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
and potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
croconates crystallize from water as dihydrates
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
but the orange potassium salt can be dehydrated to form a monohydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
.
The croconates of ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
, rubidium
Rubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
and caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
crystallize in the anhydrous form. Salts of barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, etc. are also known.
Croconic acid also forms esters such as dimethyl croconate.
History
Croconic acid and potassium croconate dihydrate were discovered by Leopold GmelinLeopold Gmelin
Leopold Gmelin was a German chemist.Gmelin was the son of Johann Friedrich Gmelin. He studied medicine and chemistry at Göttingen, Tübingen and Vienna, and in 1813 began to lecture on chemistry at Heidelberg, where in 1814 he was appointed extraordinary-, and in 1817 ordinary-, professor of...
in 1825, who named the compounds from Greek
κρόκος meaning "saffron" or "egg yolk". The structure of ammonium croconate was determined by Baenziger et al. in 1964. The structure of K2C5O5 . 2H2O was determined by J. D. Dunitz in 2001
Structure
In the solid-state, croconic acid has a peculiar structure consisting of pleated strips, each "page" of the strip being a planar ring of 4 molecules of C5O5H2 held together by hydrogen bondHydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
s. In dioxane it has a large dipole moment of
9 to 10 D
Debye
The debye is a CGS unit of electric dipole momentElectric dipole moment is defined as charge times displacement: Historically the debye was defined as the dipole moment resulting from two charges of opposite sign but an equal magnitude of 10-10 statcoulomb10-10 statcoulomb is approximately 0.2083...
, while the free molecule is estimated to have a dipole of 7 to 7.5 D. The solid is ferroelectric with a Curie point
Curie point
In physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
above 400 K, indeed the organic crystal with the highest spontaneous polarization (about 20 μC cm−2). This is due to proton transfer between adjacent molecules in each pleated sheet, rather than molecular rotation.
In the solid alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
salts, the croconate anions and the alkali cations form parallel columns. In the mixed salt K3(HC5O5)(C5O5)·2H2O, which formally contains both the croconate dianion and the hydrogencroconate monoanion , the hydrogen is shared equally by two adjacent croconate units.
Salts of the croconate anion and its derivatives are of interest in supramolecular chemistry
Supramolecular chemistry
Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...
research because of their potential for π-stacking effects, where the delocalized electrons of two stacked croconate anions interact.
The croconate anion forms hydrate
Hydrate
Hydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
d crystalline coordination compounds with divalent cations of transition metals, with general formula M(C5O5)·3H2O; where M stands for copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
(yielding a brown solid), iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
(dark purple), zinc
Zinc
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...
(yellow), nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
(green), manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
(dark green), or cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
(purple). These complexes all have the same orthorombic crystal structure, consisting of chains of alternating croconate and metal ions. Each croconate is bound to the preceding metal by one oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
atom, and to the next metal through its two opposite oxygens, leaving two oxygens unbound. Each metal is bound to three croconate oxygens and to one water molecule. Calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
also forms a compound with the same formula (yellow) but the structure appears to be different.
The croconate anion also forms compounds with trivalent cations such as aluminum (yellow), chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
(brown), and iron (purple). These compounds also include hydroxyl groups as well as hydration water and have a more complicated crystal strcuture. No indication was found of sandwich-type bonds between the delocalized electrons and the metal (as are seen in ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
, for example), but the anion can form metal complexes with a large variety of bonding patterns, involving from only one to all five of its oxygen atoms.
See also
- Croconate violetCroconate violetCroconate violet or 1,3-biscroconate 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 croconate oxocarbon anion through the replacement of two oxygen atoms by dicyanomethylene groups =C2...
- Croconate blueCroconate blueCroconate blue or 1,2,3-triscroconate is a divalent anion with chemical formula or 32−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion through the replacement of three oxygen atoms by dicyanomethylene groups =C2...
- 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....
- Squaric acidSquaric acidSquaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula 424....
- 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...
- CyclopentanepentoneCyclopentanepentoneCyclopentanepentone, also known as leuconic acid, is a hypothetical organic compound with formula C5O5, the fivefold ketone of cyclopentane...
(leuconic acid)