Thequinones are a class oforganic compounds that are formally "derived fromaromatic compounds [such asbenzene ornaphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with any necessary rearrangement ofdouble bonds", resulting in "a fullyconjugated cyclicdione structure".[1][2][3]The archetypical member of the class is1,4-benzoquinone or cyclohexadienedione, often called simply "quinone" (thus the name of the class). Other important examples are1,2-benzoquinone (ortho-quinone),1,4-naphthoquinone and9,10-anthraquinone.
The name is derived from that ofquinic acid (with the suffix "-one" indicating a ketone), since it is one of the compounds obtained upon oxidation of quinic acid.[4] Quinic acid, likequinine is obtained fromcinchona bark, calledquinaquina in the indigenous languages of Peruvian tribes.
Quinones are commonly seen inchromophores.[5] They are partly responsible for the yellowing ofpaper; the oxidation of lignin produces ketone derivatives and quinone derivatives, both of which are chromophores.[6]
Quinones are oxidized derivatives of aromatic compounds and are often readily made from reactive aromatic compounds withelectron-donating substituents such asphenols andcatechols, which increase the nucleophilicity of the ring and contributes to the largeredox potential needed to break aromaticity. (Quinones are conjugated but not aromatic). Quinones are electrophilicMichael 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 termquinone is also used more generally for a large class of compounds formally derived from aromatic quinones through replacement of somehydrogen atoms by other atoms or radicals.
Quinones form polymers by formation of hydrogen bonds with ρ-hydroquinone.[7]
Quinones areoxidizing agents, sometimes reversibly so. Relative tobenzoquinone, more strongly oxidizing quinones includechloranil and2,3-dichloro-5,6-dicyano-1,4-benzoquinone (also known as DDQ).[8]
The oxidizing power of quinones is enhanced by the presence of acids.[9] In acidic conditions, quinone undergoes two-electron and two-proton reduction tohydroquinone.
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. Inaprotic media, quinones undergo two-step reduction without protons.[10] In the first step, a short-livedsemiquinone intermediate is formed. In the second step, the semiquinone is reduced into a quinone dianion.
9,10-Anthraquinone-2,7-disulphonic acid (AQDS) a quinone similar to one found naturally inrhubarb has been used as a charge carrier in metal-freeflow batteries.[11]
Quinones undergo addition reaction to form 1,4-addition products.[12] An example of 1,4-addition reaction is the addition ofhydrogen chloride to form chlorohydroquinone:
Quinones can undergoDiels–Alder reactions.[12] The quinone acts as the dienophile and reacts with a diene at a carbon-carbon double bond.
InDiels–Alder reactions quinones are used as dienophiles. Historically importantsyntheses includecholesterol,cortisone,morphine, andreserpine.[13]
A large scale industrial application of quinones is for the production ofhydrogen peroxide. 2-Alkylanthraquinones are hydrogenated to the corresponding hydroquinones (quinizarins), which then transferH
2 to oxygen:
in this way, several million metric tons ofH
2O
2 are produced annually.[14]
1,4-Naphthoquinone, derived by oxidation of naphthalene withchromium trioxide.[15] It is the precursor to anthraquinone.
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 inrespiration 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.[16] Some serve as electron acceptors in electron transport chains such as those inphotosynthesis (plastoquinone,phylloquinone), andaerobic respiration (ubiquinone). Phylloquinone is also known as vitamin K1, as it is used by animals to carboxylate certain proteins, which are involved inblood coagulation,bone formation, and other processes. Conversely, the toxicity ofparacetamol is due to its metabolism to aquinone imine, which then reacts with liver proteins to cause liver failure.
The auto-oxidation of the neurotransmitterdopamine and its precursor L-Dopa generates the comparatively stable dopamine quinone which inhibits the functioning of dopamine transporter (DAT) and theTH enzyme and leads to low mitochondrialATP production.[17]
The benzoquinoneblattellaquinone is a sexpheromone incockroaches. In the spray ofbombardier beetles, hydroquinone reacts with hydrogen peroxide to produce a fiery blast of steam, a deterrent in the animal world.
Several quinones are of pharmacological interest. They form a major class of anticancer cytotoxins. One example isdaunorubicin, which is antileukemic.[16] Some of them show anti-tumoral activity. They embody some claims inherbal medicine. These applications include purgative (sennosides), antimicrobial and antiparasitic (rhein andsaprorthoquinone,atovaquone), anti-tumor (emodin andjuglone), inhibition ofPGE2 biosynthesis (arnebinone andarnebifuranone) and anti-cardiovascular disease (tanshinone).[18]Malbranchea cinnamomea is a thermophilic fungus, which produces a quinone antibiotic.
Another quinone-containing drug isMecarbinate (dimecarbine), made by the reaction of ethylN-methyl-β-aminocrotonate with para-benzoquinone. Others includeAmendol,Oxyphemedol,Phemedol all in FR5142 (M) ― 1967-06-05.[clarification needed] Note: These are all indoles made via theNenitzescu indole synthesis. The antineoplasticApaziquone.
Benzoquinone compounds are a metabolite ofparacetamol.[19]
Many natural and artificial coloring substances (dyes andpigments) are quinone derivatives, for instancelawsone is the active dye compound inhenna. They are second only toazo dyes in importance as dyestuffs, with particular emphasis on blue colors.Alizarin (1,2-dihydroxy-9,10-anthraquinone), extracted from themadder plant, was the first natural dye to be synthesized from coal tar.
A commercial application of quinones is inblack-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.[20]
Quinones are commonly named with a prefix that indicates the parent aromatic hydrocarbon ("benzo-" for benzene, "naphtho-" for naphthalene, "anthra-" foranthracene, 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").
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