Tetrahydroxy-1,4-benzoquinone


Tetrahydroxy-1,4-benzoquinone, also called tetrahydroxy-p-benzoquinone, tetrahydroxybenzoquinone, or tetrahydroxyquinone, is an organic compound with formula. Its molecular structure consists of a cyclohexadiene ring with four hydroxyl groups and two ketone groups in opposite positions.
The compound gives a light red solution in water, and crystallizes as the glistening bluish-black dihydrate.
The compound can be synthesized from glyoxal or from myo-inositol, a natural compound widely present in plants. THBQ forms an adduct with 4,4′-bipyridine in a 2:3 ratio.

Salts of THBQ

Like most phenols, THBQ is acidic and easily loses the four hydrogen ions from the hydroxyl groups, yielding anions such as and. The latter is symmetric and aromatic, as the double bonds and negative charges are evenly distributed over the six CO groups.
The calcium salt is the dark purple pigment produced from inositol by Chromohalobacter beijerinckii in the fermentation of salt beans, already noted by T. Hof in 1935.
The dark purple and insoluble dipotassium salt 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.
The black tetrapotassium salt 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. Partial oxidation of affords a green, strongly paramagnetic solid, conjectured to be, and complete oxidation yields potassium rhodizonate.
The greenish-black sodium salt was described by Fatiadi and Sanger in 1962.
The dark-violet lithium salt has been proposed as an electrode material for batteries as it can be oxidized to the rhodizonate and reduced to the hexahydroxybenzene salt. In the absence of oxygen, is stable to about 450 °C and then decomposes leaving a residue of lithium carbonate. Indeed, the rhodizonate appears to disproportionate at about 400 °C into and cyclohexanehexone that promptly decomposes into carbon monoxide, carbon dioxide, and carbon. forms a hydrate that loses its water at about 250 °C.