Quinone


The quinones are a class of organic compounds that are formally "derived from aromatic compounds by conversion of an even number of –CH= groups into –C– groups with any necessary rearrangement of double bonds", resulting in "a fully conjugated cyclic dione structure". The class includes some heterocyclic compounds.
The archetypical member of the class is 1,4-benzoquinone or cyclohexadienedione, often called simply "quinone". Other important examples are 1,2-benzoquinone, 1,4-naphthoquinone and 9,10-anthraquinone.

Properties

Quinones are oxidized derivatives of aromatic compounds and are often readily made from reactive aromatic compounds with electron-donating substituents such as phenols and catechols, which increase the nucleophilicity of the ring and contributes to the large redox potential needed to break aromaticity.. Quinones are electrophilic Michael acceptors stabilised by conjugation. Depending on the quinone and the site of reduction, reduction can either rearomatise the compound or break the conjugation. Conjugate addition nearly always breaks the conjugation.
The term quinone is also used more generally for a large class of compounds formally derived from aromatic quinones through replacement of some hydrogen atoms by other atoms or radicals.

Occurrence and uses

Production of hydrogen peroxide

A large scale industrial application of quinones is for the production of hydrogen peroxide. 2-Alkylanthraquinones are hydrogenated to the corresponding hydroquinones, which then transfer to oxygen:
in this way, several billion kilograms of are produced annually.

Biochemistry

Derivatives of quinones are common in biologically active molecules. They are conjectured to occur in all respiring organisms. Some serve as electron acceptors in electron transport chains such as those in photosynthesis, and aerobic respiration. Phylloquinone is also known as Vitamin K1 as it is used by animals to carboxylate certain proteins, which are involved in blood coagulation, bone formation, and other processes. Conversely, the toxicity of paracetamol is due to its metabolism to a quinone imine, which then reacts with liver proteins to cause liver failure.
In the spray of bombardier beetles, hydroquinone reacts with hydrogen peroxide to produce a fiery blast of steam, a deterrent in the animal world.
The auto-oxidation of the neurotransmitter dopamine and its precursor L-Dopa generates the comparatively stable dopamine quinone which inhibits the functioning of dopamine transporter and the TH enzyme leading to low mitochondrial ATP production.

Medicinal

Several quinones are of pharmacological interest. They form a major class of cytotoxins, used in the fight against cancers. One example is daunorubicin, which is antileukemic. Some of them show anti-tumoral activity. They embody some claims in herbal medicine. These applications include purgative, antimicrobial and antiparasitic, anti-tumor, inhibition of PGE2 biosynthesis and anti-cardiovascular disease.

Dyes

Many natural and artificial coloring substances are quinone derivatives. They are second only to azo dyes in importance as dyestuffs, with particular emphasis on blue colors. Alizarin, extracted from the madder plant, was the first natural dye to be synthesized from coal tar.

Reagents in organic chemistry

Benzoquinone is used in organic chemistry as an oxidizing agent. Strongly oxidizing quinones include chloranil and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Battery charge carrier

9,10-Anthraquinone-2,7-disulphonic acid a quinone similar to one found naturally in rhubarb has been used as a charge carrier in metal-free flow batteries.

Nomenclature

Quinones are commonly named with a prefix that indicates the parent aromatic hydrocarbon and the "-quinone" suffix. Infix multipliers "-di-", "-tri-", "-tetra-" are used when there are 4, 6, 8 carbonyls. The position of the carbonyl groups can be indicated before the prefix or after it.