Croconate blue or 1,2,3-tris(dicyanomethylene)croconate is a

divalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of a ...

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 conve ...

with

chemical formula In chemistry, 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, ...

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 In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers ''x'', ''y'', and ''n''. The most common oxocarbon anions are carbonate, , and oxalat ...

through the replacement of three

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

atoms by dicyanomethylene

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...

s =C(−C≡N)₂. The term Croconate Blue as a dye name specifically refers to the dipotassium salt .

History and synthesis

The anion was synthesized and characterized by A. Fatiadi in 1978, together with croconate violet. He obtained the corresponding acid, croconate acid blue by treating

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 sensitive ...

with

malononitrile Malononitrile is an organic compound nitrile with the formula . It is a colorless or white solid. It can be prepared by dehydration of cyanoacetamide. Malononitrile is relatively acidic, with a p''K''a of 11 in water. This allows it to be use ...

in water solution at 80–90 °C. Alexander J. Fatiadi (1978), "Synthesis of 1,3-(dicyanomethylene)croconate salts. New bond-delocalized dianion, Croconate Violet". ''Journal of the American Chemical Society'', volume 100 issue 8, pages 2586–2587. Alexander J. Fatiadi (1980), "Pseudooxocarbons. Synthesis of 1,2,3-tris(dicyanomethylene)croconate salts. A new bond-delocalized dianion, croconate blue". ''Journal of Organic Chemistry'' volume 45, pages 1338–1339.

Properties

Croconate acid blue

Croconate acid blue is strongly acidic (p''K''_a1 ≈ 1). It crystallizes from water as a purple sesquihydrate · and gives a red solution in acetone or ethanol, but deep blue in water. It hydrolyzes slowly in water to give croconic acid violet. Extended heating in water produces deep green plates, apparently a polymer. Croconate acid blue retains the aromatic character and some other properties of the croconate anion. Lawrence M. Doane, Alexander J. Fatiadi (1981) "Electrochemical Oxidation of Croconate Salts; Evidence of the Chemical Equivalence of the Carbonyl Oxygen Atom and the Dicyanomethylene Group"

Croconate blue dianion

Unlike croconate violet, the croconate blue dianion is not planar; the three dicyanomethylene groups are twisted by about 30 degrees from the ring plane. The dipotassium salt is obtained from the acid by treatment with

potassium methoxide Potassium methoxide is the alkoxide of methanol with the counterion potassium and is used as a strong base and as a catalyst for transesterification, in particular for the production of biodiesel. Preparation The preparation of potassium methox ...

from water as a green-blue trihydrate, that slowly loses water converting to the dihydrate. The water solutions have an intense blue color. It quickly converts to croconate violet when treated with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...

. It is a weak semiconductor with

conductivity Conductivity may refer to: *Electrical conductivity, a measure of a material's ability to conduct an electric current **Conductivity (electrolytic), the electrical conductivity of an electrolyte in solution ** Ionic conductivity (solid state), ele ...

10⁻⁷ S cm⁻¹ at room temperature. The bis(

tetramethylammonium Tetramethylammonium (TMA) or (Me4N+) is the simplest quaternary ammonium cation, consisting of four methyl groups attached to a central nitrogen atom, and is isoelectronic with neopentane. It is positively charged and can only be isolated in assoc ...

) salt ((CH₃)₄)₂ is green-blue and crystallizes as the dihydrate.

References

{{reflist Dyes Cyclopentenes

History and synthesis

Properties

Croconate acid blue

Croconate blue dianion

See also

References