Tetrahydroxy-1,4-benzoquinone

Tetrahydroxy-1,4-benzoquinone[1]
IUPAC name 2,3,5,6-tetrahydroxycyclohexa-2,5-diene-1,4-dione
Other names Tetroquinone
Identifiers
CAS number	319-89-1 Y
	Jmol-3D images	Image 1
SMILES O=C(C(O)=C1O)C(O)=C(O)C1=O
Properties
Molecular formula	C6H4O6
Molar mass	172.09 g mol−1
Appearance	Blue-black crystals
Density	2.609 g/cm3
Boiling point	370.6 °C @ 760 mmHg
Solubility in water	Slightly soluble in cold water
Hazards
Flash point	192.1 °C; 377.8 °F; 465.2 K
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Tetrahydroxy-1,4-benzoquinone, also called tetrahydroxy-p-benzoquinone, tetrahydroxybenzoquinone, or tetrahydroxyquinone (THBQ, THQ), is an organic compound with formula C₆O₂(OH)₄. Its molecular structure consists of a cyclohexadiene ring with four hydroxyl groups and two ketone groups in opposite (para) positions.

The compound gives a light red solution in water,^[2] and crystallizes as the glistening bluish-black (but non-conducting) dihydrate C₆O₂(OH)₄·2H₂O.^[2][3]

The compound can be synthesized from glyoxal^[4] or from myo-inositol (phytic acid), a natural compound widely present in plants.^[5] THBQ forms an adduct with 4,4'-bipyridine in the 2:3 ratio.^[6]

Salts of THBQ

Like most phenols, THBQ is acidic and easily loses the four H⁺ ions from the hydroxyl groups, yielding anions such as C₆H₂O₆²⁻ and C₆O₆⁴⁻. The latter is symmetric and aromatic, as the double bonds and negative charges are evenly distributed over the six CO groups.

The calcium salt Ca₂C₆O₆ is the dark purple pigment produced from inositol by Chromohalobacter beijerinckii in the fermentation of salt beans, already noted by T. Hof in 1935.^{[3][7][8][9][10]}

The dark purple and insoluble dipotassium salt K₂C₆H₂O₆ was prepared by Preisler and Berger in 1942, by oxidizing inositol with nitric acid and reacting the result with potassium carbonate in the presence of oxygen. Reaction of this salt with hydrochloric acid affords THBQ in good yield.^[11]

The black tetrapotassium salt K₄C₆O₆ was prepared by West and Niu in 1962, by reacting THBQ with potassium methoxide in methanol. The salt is diamagnetic and the infrared spectrum suggests that the C-C and C-O distances are all equal, with the ring slightly distorted in the "chair" conformation.^[12] Partial oxidation of K₄C₆O₆ affords a green strongly paramagnetic solid, conjectured to be (K⁺)₃ C₆O₆³⁻, and complete oxidation yields potassium rhodizonate (K⁺)₂ C₆O₆²⁻.^[12]

The greenish-black sodium salt Na₄C₆O₆ was described by Fatiadi and Sanger in 1962.^[4]

The dark-violet lithium salt Li₄C₆O₆ has been proposed as an electrode material for batteries as it can be oxidized to the rhodizonate Li₂C₆O₆ and reduced to the hexahydroxybenzene salt Li₆C₆O₆.^[5] In the absence of oxygen, Li₄C₆O₆ is stable to about 450 °C and then decomposes leaving a residue of carbonate Li₂CO₃.^[5] Indeed, the rhodizonate appears to disproportionate at about 400 °C into Li₄C₆O₆ and cyclohexanehexone C₆O₆ that promptly decomposes to carbon monoxide, carbon dioxide, and carbon.^[5] Li₂C₆O₆ forms a hydrate Li₄C₆O₆·2H₂O that loses its water at about 250 °C.^[5]

References