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{{short description|Compounds having a fully conjugated cyclic dione structure}}
{{Redirect-distinguish|Quinones|Quiñónez{{!}}Quiñones|Quinine|Quinoline|Hydroquinone}}
The '''quinones''' are a class of [[organic compound]]s that are formally "derived from [[aromatic compounds]] [such as [[benzene]] or [[naphthalene]]] by conversion of an even number of –CH=  groups into –C(=O)– groups with any necessary rearrangement of [[double bond]]s", resulting in "a fully [[Conjugated system|conjugated]] cyclic [[diketone|dione]] structure".<ref>{{GoldBookRef| title=Quinones| file=Q05015}}</ref><ref>{{cite book |doi=10.1002/9780470772119|title=The Quinonoid Compounds: Vol. 1 (1988)|year=1988|isbn=978-0-470-77211-9|editor1-last=Patai|editor1-first=Saul|editor2-last=Rappoport|editor2-first=Zvi}}</ref><ref>{{cite book |doi=10.1002/9780470772126|title=The Quinonoid Compounds: Vol. 2 (1988)|year=1988|isbn=978-0-470-77212-6|editor1-last=Patai|editor1-first=Saul|editor2-last=Rappoport|editor2-first=Zvi}}</ref>
The archetypical member of the class is [[1,4-benzoquinone]] or cyclohexadienedione, often called simply "quinone" (thus the name of the class). Other important examples are [[1,2-benzoquinone]] ('''''ortho''-quinone'''), [[1,4-naphthoquinone]] and [[anthraquinone|9,10-anthraquinone]].

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.<ref>{{Cite book|url=https://books.google.com/books?id=zfnmAAAAMAAJ&dq=quinone&pg=PA152|title=The Chemical News and Journal of Physical Science|date=1773|publisher=Griffin, Bohn and Company|language=en}}</ref> Quinic acid, like [[quinine]] is obtained from [[cinchona bark]], called [[wikt:quinaquina|quinaquina]] in the indigenous languages of Peruvian tribes.

Quinones are commonly seen in [[chromophores]].<ref>{{Cite web |date=2019-09-24 |title=2.3: UV-Visible Spectroscopy of Organic Compounds |url=https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_Structure_and_Reactivity_in_Organic_Biological_and_Inorganic_Chemistry_(Schaller)/Structure_and_Reactivity_in_Organic_Biological_and_Inorganic_Chemistry_II%3A_Practical_Aspects_of_Structure_-_Purification_and_Spectroscopy/02%3A_Ultraviolet-Visible_Spectroscopy/2.03%3A_UV-Visible_Spectroscopy_of_Organic_Compounds |access-date=2025-09-16 |website=Chemistry LibreTexts |language=en}}</ref> They are partly responsible for the yellowing of [[paper]]; the oxidation of lignin produces ketone derivatives and quinone derivatives, both of which are chromophores.<ref>{{Cite journal |last=Carter |first=Henry A. |date=1996-11-01 |title=The Chemistry of Paper Preservation: Part 2. The Yellowing of Paper and Conservation Bleaching |journal=Journal of Chemical Education |volume=73 |issue=11 |page=1068 |doi=10.1021/ed073p1068 |issn=0021-9584}}</ref>

==Properties==
Quinones are oxidized derivatives of aromatic compounds and are often readily made from reactive aromatic compounds with [[electron-donating group|electron-donating substituents]] such as [[phenols]] and [[catechol]]s, 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 [[Michael acceptor]]s 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.

<gallery class="skin-invert-image">
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|9,10-Anthraquinone]]
</gallery>

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.

<gallery  class="skin-invert-image">>
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|[[2,3-Dichloro-5,6-dicyano-1,4-benzoquinone|DDQ]], a reagent in organic chemistry
File:Daunorubicin2DACS.svg|[[Daunorubicin]], an anticancer drug
</gallery>

== Reactions ==
Quinones form polymers by formation of hydrogen bonds with ρ-hydroquinone.<ref>{{cite journal |doi=10.1107/S0567740868002451 |title=On the refinement of the crystal structures of phenoquinone and monoclinic quinhydrone |date=1968 |last1=Sakurai |first1=T. |journal=Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry |volume=24 |issue=3 |pages=403–412 |bibcode=1968AcCrB..24..403S }}</ref>
=== Reduction ===
Quinones are [[Oxidizing agent|oxidizing agents]], sometimes reversibly so. Relative to [[benzoquinone]], more strongly oxidizing quinones include [[chloranil]] and [[2,3-Dichloro-5,6-dicyano-1,4-benzoquinone|2,3-dichloro-5,6-dicyano-1,4-benzoquinone]] (also known as DDQ).<ref>{{JerryMarch}}</ref>

The oxidizing power of quinones is enhanced by the presence of acids.<ref>{{Cite journal |last1=Guin |first1=Partha Sarathi |last2=Das |first2=Saurabh |last3=Mandal |first3=P. C. |date=2011-03-16 |title=Electrochemical Reduction of Quinones in Different Media: A Review |journal=International Journal of Electrochemistry |language=en |volume=2011 |article-number=e816202 |doi=10.4061/2011/816202 |issn=2090-3529|doi-access=free }}</ref> In acidic conditions, quinone undergoes two-electron and two-proton reduction to [[hydroquinone]].
[[File:QuinoneAcidicReduction.png|center|thumb|class=skin-invert-image|247x247px|Reduction of quinone in an acidic, buffered media into hydroquinone]]

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.<ref>{{Cite journal |last1=René |first1=Alice |last2=Evans |first2=Dennis H. |date=2012-07-12 |title=Electrochemical Reduction of Some o -Quinone Anion Radicals: Why Is the Current Intensity so Small? |url=https://pubs.acs.org/doi/10.1021/jp3038335 |journal=The Journal of Physical Chemistry C |language=en |volume=116 |issue=27 |pages=14454–14460 |doi=10.1021/jp3038335 |issn=1932-7447|url-access=subscription }}</ref> In the first step, a short-lived [[semiquinone]] 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 in [[rhubarb]] has been used as a charge carrier in metal-free [[flow battery|flow batteries]].<ref>{{cite journal|title=A metal-free organic-inorganic aqueous flow battery|journal=Nature|date=9 January 2014|volume=505|issue=7482|pages=195–198|doi=10.1038/nature12909|pmid=24402280|bibcode=2014Natur.505..195H|url=https://dash.harvard.edu/bitstream/handle/1/11688785/Nature_paper_website_version.pdf?sequence=4|last1=Huskinson|first1=Brian|last2=Marshak|first2=Michael P.|last3=Suh|first3=Changwon|last4=Er|first4=Süleyman|last5=Gerhardt|first5=Michael R.|last6=Galvin|first6=Cooper J.|last7=Chen|first7=Xudong|last8=Aspuru-Guzik|first8=Alán|last9=Gordon|first9=Roy G.|last10=Aziz|first10=Michael J.|s2cid=4459692|archive-date=24 August 2021|access-date=24 August 2021|archive-url=https://web.archive.org/web/20210824111753/https://dash.harvard.edu/bitstream/handle/1/11688785/Nature_paper_website_version.pdf?sequence=4}}</ref>

=== Addition ===
Quinones undergo addition reaction to form 1,4-addition products.<ref name=":0">{{Citation |last1=Smith |first1=P. W. G. |date=1969-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780080129488500103 |work=Aromatic Chemistry |pages=144–175 |editor-last=Smith |editor-first=P. W. G. |publisher=Pergamon |language=en |doi=10.1016/b978-0-08-012948-8.50010-3 |isbn=978-0-08-012948-8 |access-date=2022-11-17 |last2=Tatchell |first2=A. R. |title=Aromatic Alcohols and Carbonyl Compounds |editor2-last=Tatchell |editor2-first=A. R.|url-access=subscription }}</ref> An example of 1,4-addition reaction is the addition of [[hydrogen chloride]] to form chlorohydroquinone:  
[[File:QuinoneAdditionReaction.png|center|thumb|class=skin-invert-image|240x240px|1,4-addition reaction of quinone with hydrogen chloride to produce chlorohydroquinone]]
Quinones can undergo [[Diels–Alder reaction|Diels–Alder reactions]].<ref name=":0" /> The quinone acts as the dienophile and reacts with a diene at a carbon-carbon double bond.

In [[Diels–Alder reaction]]s quinones are used as dienophiles. Historically important [[total synthesis|syntheses]] include [[cholesterol total synthesis|cholesterol]], [[cortisone]], [[total synthesis of morphine and related alkaloids|morphine]], and [[reserpine]].<ref>{{Cite journal |last1=Nawrat |first1=Christopher C. |last2=Moody |first2=Christopher J. |date=2014-02-17 |title=Quinones as Dienophiles in the Diels–Alder Reaction: History and Applications in Total Synthesis |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.201305908 |journal=Angewandte Chemie International Edition |language=en |volume=53 |issue=8 |pages=2056–2077 |doi=10.1002/anie.201305908|pmid=24446164 |bibcode=2014ACIE...53.2056N |s2cid=1362687 }}</ref>

== Occurrence and uses ==
=== Production of hydrogen peroxide ===
{{Main|Anthraquinone process}}
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 {{chem|H|2}} to oxygen:

: dihydroanthraquinone  +  {{chem|O|2}}   →   [[anthraquinone]]  +  {{chem|H|2|O|2}}

in this way, several million metric tons of {{chem|H|2|O|2}} are produced annually.<ref>Gustaaf Goor, Jürgen Glenneberg, Sylvia Jacobi "Hydrogen Peroxide" in Ullmann's Encyclopedia of Industrial Chemistry 2007, Wiley-VCH, Weinheim. {{doi| 10.1002/14356007.a13_443.pub2}}.</ref>

1,4-[[Naphthoquinone]], derived by oxidation of naphthalene with [[chromium trioxide]].<ref>{{ OrgSynth | author = Braude E. A. | author2 = Fawcett, J. S. | title = 1,4-Naphthoquinone | year = 1953 | volume = 33 | page = 50 | collvol = 4 | collvolpages = 698 | doi=10.15227/orgsyn.033.0050}}</ref> It is the precursor to anthraquinone.

=== 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 [[Cellular respiration|respiration]] apparatus. [[Plastoquinone]] is a redox relay involved in photosynthesis.  [[Pyrroloquinoline quinone]] is another biological redox cofactor.

[[File:Ubiquinone.svg|thumb|class=skin-invert-image|right|Ubiquinones, as their name implies, are ubiquitous in living creatures, being components of respiratory apparatus.]]
[[File:Gentisyl quinone isovalerate.png|class=skin-invert-image|Blattellaquinone, a sex pheromone in cockroaches|thumb|right]]
Quinones are conjectured to occur in all respiring organisms.<ref name=Obrien>{{cite journal|doi=10.1016/0009-2797(91)90029-7|title=Molecular mechanisms of quinone cytotoxicity|year=1991|last1=O'Brien|first1=P.J.|journal=Chemico-Biological Interactions|volume=80|issue=1|pages=1–41|pmid=1913977 |bibcode=1991CBI....80....1O }}</ref> Some serve as electron acceptors in electron transport chains such as those in [[photosynthesis]] ([[plastoquinone]], [[phylloquinone]]), and [[aerobic respiration]] ([[ubiquinone]]). Phylloquinone is also known as vitamin K<sub>1</sub>, 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 [[NAPQI|quinone imine]], which then reacts with liver proteins to cause liver failure.

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 [[Tyrosine hydroxylase|TH]] enzyme and leads to low mitochondrial [[Adenosine triphosphate|ATP]] production.<ref>{{Cite journal|title=Molecular Effects of L-dopa Therapy in Parkinson's Disease|last1=Dorszewska|first1=Jolanta|last2=Prendecki|first2=Michal|date=2014-01-31|journal=Current Genomics|volume=15|issue=1|pages=11–17|language=en|doi=10.2174/1389202914666131210213042|pmc=3958954|pmid=24653659|last3=Kozubski|first3=Margarita Lianeri and Wojciech}}</ref>

The  benzoquinone [[blattellaquinone]] is a sex [[pheromone]] in [[cockroach]]es. In the spray of [[bombardier beetle]]s, 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.<ref name=Obrien/> Some of them show anti-[[tumor]]al activity. They embody some claims in [[herbal medicine]]. These applications include purgative ([[sennosides]]), antimicrobial and antiparasitic ([[rhein (molecule)|rhein]] and [[saprorthoquinone]], [[atovaquone]]), anti-tumor ([[emodin]] and [[juglone]]), inhibition of [[PGE2]] biosynthesis ([[arnebinone]] and [[arnebifuranone]]) and anti-[[cardiovascular disease]] ([[Salvia miltiorrhiza|tanshinone]]).<ref>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.</ref>  [[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.{{clarify|date=July 2018}} Note: These are all indoles made via the [[Nenitzescu indole synthesis]]. The antineoplastic [[Apaziquone]].

Benzoquinone compounds are a metabolite of [[paracetamol]].<ref>{{Cite journal
 | doi = 10.1073/pnas.81.5.1327
 | last1 = Dahlin | first1 = D. C.
 | last2 = Miwa | first2 = G. T.
 | last3 = Lu | first3 = A. Y.
 | last4 = Nelson | first4 = S. D.
 | title = N-acetyl-p-benzoquinone imine: a cytochrome P-450-mediated oxidation product of acetaminophen
 | journal = Proceedings of the National Academy of Sciences of the United States of America
 | volume = 81
 | issue = 5
 | pages = 1327–1331
 | year = 1984
 | pmid = 6424115
 | pmc = 344826
| bibcode = 1984PNAS...81.1327D | doi-access = free }}</ref>

=== Dyes ===
Many natural and artificial coloring substances ([[dye]]s and [[pigment]]s) 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|madder]] plant, was the first natural dye to be synthesized from coal tar.

=== Photography ===
A commercial application of quinones is in [[Monochrome photography|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.<ref>{{Cite book |last1=Brown |first1=William Henry |title=Organic chemistry |last2=Iverson |first2=Brent L. |last3=Anslyn |first3=Eric V. |last4=Foote |first4=Christopher S. |date=2018 |publisher=Cengage Learning |isbn=978-1-305-58035-0 |edition=8th |location=Boston, MA |oclc=974377227}}</ref>

== 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]]{{snd}} where one O is replaced by C
* [[Xylylene]]{{snd}} where both O's are replaced by C's
* Quinone imine{{snd}} where one O is replaced by N, illustrated by [[NAPQI]]
* Quinone diimine{{snd}} where both O's are replaced by N's, illustrated by the antiseptic [[ambazone]]
* Azaxylylene{{snd}} where both O's are replaced by one N and one C, illustrated by various fuchsine dyes like [[pararosaniline]]

== References ==
{{Reflist}}

== External links ==
* {{MeshName|Quinones}}
{{Plant pigments}}{{Authority control}}

[[Category:Quinones| ]]
[[Category:Enones]]