Rhodizonic acid is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

with formula or . It can be seen as a twofold

enol In organic chemistry, alkenols (shortened to enols) are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene ( olefin) with a hydroxyl group attached to one end of the alkene double bond (). T ...

and fourfold

ketone In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...

cyclohexene Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes. Cyclohexene is not very stable upon long term storage with exposure to light ...

, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone. Rhodizonic acid is usually obtained in the form of a

dihydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...

. The latter is actually 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, where two of the original ketone groups are replaced by two pairs of geminal diols. The orange to deep-red and highly

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance' ...

anhydrous acid can be obtained by low-pressure sublimation of the dihydrate. Like many other enols, rhodizonic acid can lose the hydrogen cations H⁺ from the hydroxyls (p''K''_a1 = , p''K''_a2 = at 25 °C), yielding the hydrogenrhodizonate anion and the rhodizonate anion . The latter is aromatic and symmetric, as the

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

and the negative charges are

delocalized In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref> The term delocalization is general and can have slightly dif ...

and evenly distributed over the six CO units. Rhodizonates tend to have various shades of red, from yellowish to purplish. Rhodizonic acid has been used in chemical assays for

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

, and other metals. In particular, the sodium rhodizonate test can be used to detect

gunshot residue Gunshot residue (GSR), also known as cartridge discharge residue (CDR), gunfire residue (GFR), or firearm discharge residue (FDR), consists of all of the particles that are expelled from the muzzle of a gun following the discharge of a bullet. It ...

(which contains lead) in a subject's hands, and to distinguish

arrow An arrow is a fin-stabilized projectile launched by a bow. A typical arrow usually consists of a long, stiff, straight shaft with a weighty (and usually sharp and pointed) arrowhead attached to the front end, multiple fin-like stabilizers ...

wounds from gunshot wounds for hunting regulation enforcement.

History

Rhodizonic acid was discovered by

Austrian Austrian may refer to: * Austrians, someone from Austria or of Austrian descent ** Someone who is considered an Austrian citizen, see Austrian nationality law * Austrian German dialect * Something associated with the country Austria, for example: ...

chemist Johann Heller in 1837, by analyzing the products of heating a mixture of

potassium carbonate Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and gl ...

and

charcoal Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, ...

. The name comes from

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

(''rhodizō'', "to tinge red"), on account of the color of its salts.

Chemistry

Salts

Rhodizonates tend to have various shades of red, from yellowish to purplish, in transmitted light, with a greenish luster in reflected light.

Potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

rhodizonate can be prepared with good yield and purity by oxidizing

inositol Inositol, or more precisely ''myo''-inositol, is a carbocyclic sugar that is abundant in the brain and other mammalian tissues; it mediates cell signal transduction in response to a variety of hormones, neurotransmitters, and growth factors and ...

with

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

and reacting the result with

potassium acetate Potassium acetate (CH3COOK) is the potassium salt of acetic acid. It is a hygroscopic solid at room temperature. Preparation It can be prepared by treating a potassium-containing base such as potassium hydroxide or potassium carbonate with acetic ...

in the presence of oxygen. The rhodizonate crystallizes out of the solution due to its relative insolubility in water.

Sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

rhodizonate is dark brown and stable when dry, but the aqueous solution decomposes in a few days, even in the refrigerator.

Lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...

rhodizonate is dark violet.

Oxidation and decomposition

Rhodizonic acid is a member of a chain of oxidation products: benzenehexol , tetrahydroxybenzoquinone (THBQ) , rhodizonic acid , and

cyclohexanehexone Cyclohexanehexone, also known as hexaketocyclohexane and triquinoyl, is an organic compound with formula , the sixfold ketone of cyclohexane. It is an oxide of carbon (an oxocarbon), a hexamer of carbon monoxide. The compound is expected to be ...

. Lithium rhodizonate, together with salts of THBQ and benzenehexol, has been considered for possible use in rechargeable electrical batteries. The monovalent anion has been detected in mass spectrometry experiments. Rhodizonic acid and the rhodizonate anion can lose one of the CO units to yield

croconic acid Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula or . 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 sensitiv ...

and the croconate anion , respectively, by mechanisms that are still imperfectly known. In basic solutions (pH > 10), rhodizonic acid quickly converts to the THBQ anion in the absence of oxygen, and to croconic acid in its presence. At pH 8.3 and exposure to light, solutions are stable for days in the absence of oxygen, and decompose to croconic acid and other products (possibly including

dodecahydroxycyclohexane Dodecahydroxycyclohexane is an organic chemistry, organic compound with molecular formula or or . It is a sixfold geminal diol with a cyclohexane backbone and can be regarded as a sixfold hydrate of cyclohexanehexone (). Dihydrate The dihydrate ...

) in its presence.

Structure

Acid

In solution, the acid and the hydrogenrhodizonate ion are mostly hydrated, with some of the carbonyl groups >C=O replaced by geminal hydroxyls, .

Salts

In anhydrous

rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...

rhodizonate , the rhodizonate anions are stacked in parallel columns, as are the rubidium ions. In the plane perpendicular to the columns, these are arranged as two interleaved hexagonal grids. The anions are planar. Anhydrous potassium rhodizonate has a distinct but similar structure. The anions and cations are arranged in alternate planes. Within each plane, the anions are arranged in a hexagonal grid. Each potassium ion is arranged so that it connects symmetrically to eight oxygens of four anions, two from each adjacent plane. The anions are slightly twisted in the "boat" shape (with 0.108 Å of rms deviation from mean plane). Sodium rhodizonate has the same structure, with slightly more distorted anions (0.113 Å rms) In solution, the rhodizonate anion is not hydrated.

References

{{reflist Aromatic compounds Analytical chemistry Tetraones Enediols Oxocarbons Cyclohexenols

History

Chemistry

Salts

Oxidation and decomposition

Structure

Acid

Salts

See also

References