Quinone
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A quinone or benzoquinone is generally defined as an aromatic benzene molecule containing a double ketone functional group. Benzoquinone refers to either of two cyclic isomers with the chemical formula C6H4O2: orthobenzoquinone and parabenzoquinone. Parabenzoquinone will be in equilibrium with a free radical form in which the benzene ring is in resonance. Parabenzoquinone is the oxidized form of hydroquinone. For example, an acidic Potassium iodide solution reduces a solution of benzoquinone to hydroquinone which is oxidized back with a solution of silver nitrate.
| Image:Orthobenzoquinone.png orthobenzoquinone |
Image:Parabenzoquinone.png parabenzoquinone |
The word quinone can also be used for a family of molecules having a cycle with two double bonds similar to benzoquinone.
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Biochemistry
Quinone is a common constituent of biologically relevant molecules (e.g. Vitamin K1 is phylloquinone) that serve as electron acceptors in electron transport chains such as those in Photosystems I & II of photosynthesis, and aerobic respiration. It is a strong oxidizer and because of this it is used in the spray of bombardier beetles. Hydroquinone is reacted with hydrogen peroxide to produce a fiery blast of steam, a strong deterent in the animal world.
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 [1].