Quinone

Benzoquinone, or quinone is one of the two isomers of cyclohexadienedione. It has the molecular formula C6H4O2. Orthobenzoquinone is the 1,2-dione, whereas parabenzoquinone, is the 1,4-dione.

Orthobenzoquinone is the oxidized form of catechol (1,2-dihydroxybenzene), while parabenzoquinone is the oxidized form of hydroquinone. An acidic potassium iodide solution reduces a solution of benzoquinone to hydroquinone, which is oxidized back with a solution of silver nitrate.

Quinone is also the name for the class of compounds containing either benzoquinone isomers as part of their structure. Quinones are not aromatic, but are dienes. The carbonyl groups are ketone-like.

Biochemistry
Quinone is a common constituent of biologically relevant molecules (e.g. Vitamin K1 is phylloquinone). Others serve as electron acceptors in electron transport chains such as those in Photosystems I & II of photosynthesis, and aerobic respiration. A natural example of quinones as oxidizing agents is the spray of bombardier beetles. Hydroquinone is reacted with hydrogen peroxide to produce a fiery blast of steam, a strong deterrent in the animal world. Quinones can be partially reduced to quinols.

Organic chemistry
Benzoquinone is used in organic chemistry as an oxidizing agent. Even stronger agents exist such as 2,3,5,6-tetrachloro-parabenzoquinone or p-chloranil and 2,3-dicyano-5,6-dichloro-parabenzoquinone or DDQ for short.