Quinone

Quinones (English Quinones ) are a large and important group of organic compounds that can be used as oxidation products of aromatic compounds, particularly phenols, construed. The name Chinon derives from the quinic acid whose oxidation leads inter alia to 1,4- benzoquinone. Quinones are benzene derivatives in which the substituents are replaced by double bonded oxygen to repeal the aromaticity of the ring to two carbon atoms. Substances whose chemical structure contains a quinone - element called quinoid, the structural element itself quinoid system. The reduction of the corresponding quinones provides phenols which are also called quinols.

The consistently colored quinones are oxidants, the redox potential ( etc. halogen, cyano, alkyl, hydroxyl groups) significantly changed by substituents. A distinction is 1,2 - (ortho- ) quinones (e.g. pyrroloquinolinequinone ) and 1,4 - (para- ) quinones (for example, anthraquinone ).

Formal replacement of the oxygen a quinoid carbonyl group by = NH, = NOH = N2 or = CH2 leads to quinone, quinone, and quinone quinonediazides.

Synonymously referred quinone compound, 1,4- benzoquinone, which is produced by oxidation of 1,4- dihydroxybenzene ( hydroquinone).

Among the quinones, there are many poisons, but also a wide range of vital nutrients, such as ubiquinone -10 (coenzyme Q10 ), vitamin K and pyrroloquinolinequinone.

Occurrence

In Nature quinones particularly common in dyestuffs, such as fungi, bacteria or flowers. Quinoid systems are also found in various antibiotics. Quinones are also formed by enzymatic oxidation of polyphenols. To play quinones example, in the browning apples angeschnittener an important role.

Known quinones are:

• The ubiquinone as an electron carrier in the respiratory chain
• The plastoquinones in photosynthesis
• The K vitamins ( phylloquinone, menaquinone )
• The pyrroloquinoline quinone (PQQ )
• The Tryptophantryptophylchinon ( TQQ )
• The dichlorodicyanoquinone (DDQ )
• The Hydroxynaphthochinone ( in henna and walnuts )
• Echinochrome A, adrenochrome, dopachrome

Quinones are also found in the defensive secretions of insects, especially benzo - and Toluchinone.

Representation

Quinones formed inter alia by oxidation of phenols or polycyclic aromatic hydrocarbons. This is especially cerium (IV ) compounds such as cerium (IV ) ammonium nitrate prove useful.

Reactions

Synthetic important are the (possibly photochemically induced ) cycloaddition reactions of quinones, which can lead to cyclobutane derivatives, oxetanes, cage compounds and pyrans. Olefins, for example added during exposure to the 1,2- dicarbonyl system of ortho- quinones, which functions as a hetero -1 ,3-diene system in this case (see the Diels-Alder reaction). This is among other things the formation of dioxane derivatives ( Schönberg reaction).

This in contrast to the pure substances deeply colored 1:1 mixture of 1,4- hydroquinone and 1,4- benzoquinone is called quinhydrone, a classic example of a charge-transfer complex.

Semiquinones

The reduction of quinones quinols is done in several stages on reactive free radical intermediates which semiquinones. The reduction processes with acid-base equilibria are coupled. These are in some cases quite resistant as the semiquinones of ubiquinones. The reduction of the quinone with a semiquinone form and the further reduction to the hydroquinone will be explained using the example of 1,4-benzoquinone:

Is formed from the 1,4-benzoquinone 1 after protonation and reducing the semiquinone 2 The unpaired electron is distributed over the system semiquinoid. Further reduction together with protonation leads to hydroquinone 3

Use

Quinones are used as oxidizing agents, bactericides, and analytical reagents used. They are also formed as an intermediate in dye syntheses (see alizarin ). Also in malaria drugs quinones play an important role. In industrial chemistry, they are especially important for the production of hydrogen peroxide via anthraquinone.