The quinones are a class of that are formally "derived from aromatic compounds such by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement of ", resulting in "a fully conjugated cyclic diketone structure".
The name is derived from that of quinic acid (with the suffix "-one" indicating a ketone), since it is one of the compounds obtained upon oxidation of quinic acid.
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
, which increase the nucleophilicity of the ring and contributes to the large
redox potential needed to break aromaticity. (Quinones are conjugated but not aromatic). Quinones are electrophilic
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.
File:orthobenzoquinone.svg|1,2-Benzoquinone
File:P-Benzochinon.svg|1,4-Benzoquinone
File:1,4-Naphthoquinone.svg|1,4-Naphthoquinone
File:Anthrachinon.svg|anthraquinone
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.
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File:2,3,5,6-tetrachloro-parabenzoquinone.svg|Chloranil, a reagent in organic chemistry
File:HNQ.svg|Lawsone, a dye present in the leaves of the henna plant
File:Alizaryna.svg|Alizarin, a common red dye
File:Dichlorodicyanobenzoquinone.svg|DDQ, a reagent in organic chemistry
File:Daunorubicin2DACS.svg|Daunorubicin, an anticancer drug
Reactions
Quinones form polymers by formation of hydrogen bonds with ρ-hydroquinone.
Reduction
Quinones are
Oxidizing agent, sometimes reversibly so. Relative to
benzoquinone, more strongly oxidizing quinones include
chloranil and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (also known as DDQ).
The oxidizing power of quinones is enhanced by the presence of acids.
In alkaline conditions, quinones undergo a reversible single-step, two-electron reduction. In neutral conditions, quinones may undergo either a one-proton, two-electron reduction or a two-electron reduction. In aprotic media, quinones undergo two-step reduction without protons.
9,10-Anthraquinone-2,7-disulphonic acid (AQDS) a quinone similar to one found naturally in rhubarb has been used as a charge carrier in metal-free flow battery.
Addition
Quinones undergo addition reaction to form 1,4-addition products.
An example of 1,4-addition reaction is the addition of hydrogen chloride to form chlorohydroquinone:
Quinones can undergo Diels–Alder reactions.
The quinone acts as the dienophile and reacts with a diene at a carbon-carbon double bond.
In Diels–Alder reactions quinones are used as dienophiles. Historically important total synthesis include cholesterol, cortisone, morphine, and reserpine.
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 (quinizarins), which then transfer to oxygen:
- dihydroanthraquinone + → anthraquinone +
in this way, several million metric tons of are produced annually.[Gustaaf Goor, Jürgen Glenneberg, Sylvia Jacobi "Hydrogen Peroxide" in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim. .]
1,4-Naphthoquinone, derived by oxidation of naphthalene with chromium trioxide.
Biochemistry
Numerous quinones are significant roles in biology. Vitamin K, which is involved in coagulation of blood, is a quinone.
Ubiquinone-10 is a naturally occurring 1,4-benzoquinone involved in respiration apparatus.
Plastoquinone is a redox relay involved in photosynthesis. Pyrroloquinoline quinone is another biological redox cofactor.
Quinones are conjectured to occur in all respiring organisms.
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 (DAT) and the TH enzyme and leads to low mitochondrial ATP production.
The benzoquinone blattellaquinone is a sex pheromone in . In the spray of bombardier beetles, hydroquinone reacts with hydrogen peroxide to produce a fiery blast of steam, a deterrent in the animal world.
Medical
Several quinones are of pharmacological interest. They form a major class of anticancer cytotoxins. One example is
daunorubicin, which is antileukemic.
[ Some of them show anti- activity. They embody some claims in herbal medicine. These applications include purgative (sennosides), antimicrobial and antiparasitic (rhein and saprorthoquinone, atovaquone), anti-tumor (emodin and juglone), inhibition of PGE2 biosynthesis (arnebinone and arnebifuranone) and anti-cardiovascular disease (tanshinone).][Liu H., "Extraction and Isolation of Compounds from Herbal Medicines" in 'Traditional Herbal Medicine Research Methods', ed by Willow JH Liu 2011 John Wiley and Sons, Inc.] Malbranchea cinnamomea is a thermophilic fungus, which produces a quinone antibiotic.
Another quinone-containing drug is Mecarbinate (dimecarbine), made by the reaction of ethyl N-methyl-β-aminocrotonate with para-benzoquinone. Others include Amendol, Oxyphemedol, Phemedol all in FR5142 (M) ― 1967-06-05. Note: These are all indoles made via the Nenitzescu indole synthesis. The antineoplastic Apaziquone.
Benzoquinone compounds are a metabolite of paracetamol.
Dyes
Many natural and artificial coloring substances ( and ) are quinone derivatives, for instance lawsone is the active dye compound in henna. They are second only to azo dyes in importance as dyestuffs, with particular emphasis on blue colors. Alizarin (1,2-dihydroxy-9,10-anthraquinone), extracted from the Rubia plant, was the first natural dye to be synthesized from coal tar.
Photography
A commercial application of quinones is in black-and-white photography. Black-and-white film is covered with an emulsion containing silver bromide or silver iodide crystals, which exposure to light activates. Hydroquinone is used to reduce the activated silver ions to metallic silver. During this process, hydroquinone is oxidized to quinone. All silver halide not activated by light or reduced by hydroquinone is removed, leaving a negative by deposited silver where the film had been struck by light.
Nomenclature
Quinones are commonly named with a prefix that indicates the parent aromatic hydrocarbon ("benzo-" for benzene, "naphtho-" for naphthalene, "anthra-" for anthracene, etc.) and the "-quinone" suffix. Infix multipliers "-di-", "-tri-", "-tetra-" (etc.) are used when there are 4, 6, 8 (etc.) carbonyls. The position of the carbonyl groups can be indicated before the prefix (as in "1,4,5,8-naphthodiquinone") or after it ("anthra-1,4-quinone").
Structural analogues of quinones
-
Quinone methide where one O is replaced by C
-
Xylylene where both O's are replaced by C's
-
Quinone imine where one O is replaced by N, illustrated by NAPQI
-
Quinone diimine where both O's are replaced by N's, illustrated by the antiseptic ambazone
-
Azaxylylene where both O's are replaced by one N and one C, illustrated by various fuchsine dyes like pararosaniline
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