{{Short description|1=Organic compound with a C=C–OH group}} <div class="skin-invert-image"> {{multiple image|caption_align=left|header_align=center | align = right | direction = vertical | width = 250 | header = Keto-enol tautomerism examples | image1 = enol.svg | alt1 = TBD | caption1 =3-Pentanone, a less stabilized enol (ketone left, enol right) | image2 = Enolate Resonance.svg | alt2 = TBD | caption2 = 2-Butanoate resonance forms (carbanion left, enoxide right) | image4 =AcacH.svg | alt4 = TBD | caption4 = 2,4-pentanedione, a hydrogen bond (---) stabilized enol (mono-enol left, diketone right) | image5 = Tartronaldehyde.svg | alt5 = TBD | caption5 = Tartronaldehyde, a reductone enediol (enol left, aldehyde right) }} </div> In organic chemistry, '''enols''' are a type of functional group or intermediate in organic chemistry. Formally, enols are derivatives of vinyl alcohol, with a {{chem2|C\dC\sOH}} connectivity. The term ''enol'' is an abbreviation of ''alkenol'', a portmanteau deriving from "-ene"/"alkene" and "-ol"/"alcohol".

'''Keto–enol tautomerism''' refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol. The tautomeric interconversion involves hydrogen atom movement and the reorganisation of bonding electrons.<ref name="Clayden-2012">{{cite book |last1=Clayden |first1=Jonathan |last2=Greeves |first2=Nick |last3=Warren |first3=Stuart |title=Organic chemistry |date=2012 |publisher=Oxford University Press |location=New York |isbn=978-0-19-927029-3 |pages=450–451 |edition=2nd}}</ref>

Many kinds of enols are known, but very few are stable compounds.<ref name="March" /> However, deprotonation of organic carbonyls gives enolate anions, which are important in organic reaction strategies as a strong nucleophile.

==Enolization== Organic esters, ketones, and aldehydes with an α-hydrogen ({{chem2|C\sH}} bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton ({{red|H}}) from carbon to oxygen:<ref name=March>{{cite book |author=Smith MB, March J |title=Advanced Organic Chemistry |edition=5th |publisher=Wiley Interscience |location=New York |year=2001 |pages=1218–1223 |isbn=0-471-58589-0}}</ref> :{{chem2 | RC(\dO)C{{red|H}}R′R′′ <-> RC(O{{red|H}})\dCR′R′′ }} The process does not occur intramolecularly, but requires participation of solvent or other mediators.{{Citation needed|date=November 2025}}

Strictly speaking, the conversion is a keto-enol tautomerism only in the case of ketones (neither R nor R′ hydrogen). But this name is often more generally applied to all such tautomerizations.

The keto-enol equilibrium involves movement of a double bond. If the α position of an enol is substituted (i.e., not a methyl ketone), then it is prochiral, forming a new stereocenter when in keto form. Conversely, enolization racemizes that stereocenter.{{cn|date=February 2024}}

== Occurrence and reactivity == {{See also|Carbonyl α-substitution reactions}} Usually the tautomerization equilibrium constant is so small that the enol is undetectable spectroscopically. In the equilibrium between vinyl alcohol and acetaldehyde, ''K''&nbsp;=&nbsp;[enol]/[keto]&nbsp;≈&nbsp;5.8{{x10^|-7}}.<ref name=EnolPrediction/>

The terminus of the double bond in enols is nucleophilic, a property enhanced in the case of enolate anions.<ref name=":1">{{March6th}}</ref><ref name="enolate">{{cite book |author=Manfred Braun |title=Modern Enolate Chemistry: From Preparation to Applications in Asymmetric Synthesis |publisher=Wiley-VCH |year=2015 |isbn=9783527671069 |doi=10.1002/9783527671069}}</ref> However, enolates protonate reversibly at the oxygen much faster than equilibrate to the ketone/aldehyde/etc.<ref>{{Cite journal |last=Zimmerman |first=Howard E. |date=1987-07-01 |title=Kinetic protonation of enols, enolates, and analogs. The stereochemistry of ketonization |url=https://pubs.acs.org/doi/abs/10.1021/ar00139a005 |journal=Accounts of Chemical Research |language=en |volume=20 |issue=7 |pages=263–268 |doi=10.1021/ar00139a005 |issn=0001-4842|url-access=subscription }}</ref> As many organic syntheses involve the controlled formation and reaction of enolates, enols appear transiently in great quantities during quenching.<ref name=":1" /><ref name="enolate" />

=== Stable enols === thumb|class=skin-invert-image|Diaryl-substitution stabilizes some enols.<ref>{{cite journal |title=Stable simple enols |journal=Journal of the American Chemical Society |year=1989 |doi=10.1021/ja00203a019}}</ref> Enols can be stabilized through vinylogy. Thus, very stable enols are phenols.<ref name=":0">{{Cite book |last=Clayden |first=Jonathan |title=Organic Chemistry |publisher=Oxford University Press |year=2012 |pages=456–459}}</ref>

In compounds with two (or more) carbonyls, the enol form is also stabilized through intramolecular hydrogen bonding<ref>{{Cite journal |last1=Zhou |first1=Yu-Qiang |last2=Wang |first2=Nai-Xing |last3=Xing |first3=Yalan |last4=Wang |first4=Yan-Jing |last5=Hong |first5=Xiao-Wei |last6=Zhang |first6=Jia-Xiang |last7=Chen |first7=Dong-Dong |last8=Geng |first8=Jing-Bo |last9=Dang |first9=Yanfeng |last10=Wang |first10=Zhi-Xiang |date=2013-01-14 |title=Stable acyclic aliphatic solid enols: synthesis, characterization, X-ray structure analysis and calculations |journal=Scientific Reports |language=en |volume=3 |issue=1 |pages=1058 |bibcode=2013NatSR...3E1058Z |doi=10.1038/srep01058 |issn=2045-2322 |pmc=3544012 |pmid=23320139 |doi-access=free}}</ref> and becomes dominant. The behavior of 2,4-pentanedione illustrates this effect:<ref>{{cite journal |title=Substituent Effects on Keto–Enol Equilibria Using NMR Spectroscopy |first1=Kimberly A.|last1=Manbeck|first2=Nicholas C.|last2=Boaz|first3=Nathaniel C.|last3=Bair|first4=Allix M. S.|last4=Sanders|first5=Anderson L.|last5=Marsh|year=2011 |journal=J. Chem. Educ.|volume=88|issue=10|pages=1444–1445|doi=10.1021/ed1010932 |bibcode=2011JChEd..88.1444M}}</ref> :200px|left|class=skin-invert-image{{clear-left}}

{| class="wikitable" |+ Selected enolization constants<ref name=EnolPrediction>{{cite journal |doi=10.1002/poc.3168|title=Equilibrium constants for enolization in solution by computation alone|year=2013|last1=Guthrie|first1=J. Peter|last2=Povar|first2=Igor|journal=Journal of Physical Organic Chemistry|volume=26|issue=12|pages=1077–1083|postscript=none}} See column "{{math|''p''K{{su|b=E|p=Expt}}}}" in Table&nbsp;1; values there are negative decimal logarithms of values here.</ref> !carbonyl !enol !K<sub>enolization</sub> |- |Acetaldehyde<br />{{chem2|CH3CHO}} |{{chem2|CH2\dCHOH}} |5.8{{x10^|-7}} |- |Acetone<br />{{chem2|CH3C(O)CH3}} |{{chem2|CH3C(OH)\dCH2}} |5.12{{x10^|-7}} |- |Methyl acetate<br />{{chem2|CH3CO2CH3}} |{{chem2|CH2\dCH(OH)OCH3}} |4{{x10^|-20}} |- |Acetophenone<br />{{chem2|C6H5C(O)CH3}} |{{chem2|C6H5C(OH)\dCH2}} |1{{x10^|-8}} |- |Acetylacetone<br />{{chem2|CH3C(O)CH2C(O)CH3}} |{{chem2|CH3C(O)CH\dC(OH)CH3}} |0.27 |- |Trifluoroacetylacetone<br />{{chem2|CH3C(O)CH2C(O)CF3}} |{{chem2|CH3C(O)CH\dC(OH)CF3}} |32 |- |Hexafluoroacetylacetone<br />{{chem2|CF3C(O)CH2C(O)CF3}} |{{chem2|CF3C(O)CH\dC(OH)CF3}} |~10<sup>4</sup> |- |Cyclohexa-2,4-dienone |Phenol<br />{{chem2|C6H5OH}} |>10<sup>12</sup> |}

==== Phenols ==== Phenols represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions of resorcinol and phloroglucinol involve the keto tautomers, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.<ref>{{cite journal|doi=10.1002/anie.200502588|title=Rediscovery, Isolation, and Asymmetric Reduction of 1,2,3,4-Tetrahydronaphthalene-1,4-dione and Studies of Its [Cr(CO)3] Complex|year=2006|last1=Kündig|first1=E. Peter|last2=Enríquez García|first2=Alvaro|last3=Lomberget|first3=Thierry|last4=Bernardinelli|first4=Gérald|journal=Angewandte Chemie International Edition|volume=45|issue=1|pages=98–101|pmid=16304647}}</ref> :220px|class=skin-invert-image

==Biochemistry== Keto–enol tautomerism is important in several areas of biochemistry.{{cn|date=February 2024}}

The high phosphate-transfer potential of phosphoenolpyruvate results from the fact that the phosphorylated compound is "trapped" in the less thermodynamically favorable enol form, whereas after dephosphorylation it can assume the keto form.{{cn|date=February 2024}}

The enzyme enolase catalyzes the dehydration of 2-phosphoglyceric acid to the enol phosphate ester. Metabolism of PEP to pyruvic acid by pyruvate kinase (PK) generates adenosine triphosphate (ATP) via substrate-level phosphorylation.<ref>{{cite book |last=Berg |first=Jeremy M. |author2=Tymoczko, Stryer |title=Biochemistry |year=2002 |edition=5th |publisher=W.H. Freeman and Company |location=New York |isbn=0-7167-3051-0 |url=https://archive.org/details/biochemistrychap00jere |url-access=registration }}</ref>

{| style="background: white; text-align:center;" |- | rowspan="5" | class=skin-invert-image | colspan="2" style="width:75px" | | rowspan="5" | class=skin-invert-image | colspan="2" style="width:75px" | | rowspan="5" | class=skin-invert-image |- | | H<sub>2</sub>O | ADP | ATP |- | colspan="2" style="width:75px" | 75px|class=skin-invert-image | colspan="2" style="width:75px" | 75px|class=skin-invert-image |- | | H<sub>2</sub>O | | |- | colspan="2" style="width:75px" | | colspan="2" style="width:75px" | |- |}

=== Enediols === Enediols are alkenes with a hydroxyl group on each carbon of the C=C double bond. Normally such compounds are disfavored components in equilibria with acyloins. One special case is catechol, where the C=C subunit is part of an aromatic ring. In some other cases however, enediols are stabilized by flanking carbonyl groups. These stabilized enediols are called reductones. Such species are important in glycochemistry, e.g., the Lobry de Bruyn–Van Ekenstein transformation.<ref>{{cite journal |author=Schank, Kurt |year=1972 |title=Reductones |journal=Synthesis |volume=1972 |issue=4 |pages=176–90 |doi=10.1055/s-1972-21845 |s2cid=260331550}}</ref> :[[File:Keto-Endiol-Tautomerie.svg|thumb|center|350 px|class=skin-invert-image|Hydroxyacetone tautomers (enediol center; acyloins left and right)]]

:[[Image:Ascorbic acidity3.png|thumb|center|350 px|class=skin-invert-image|Conversion of ascorbic acid (vitamin C) to an enolate. Enediol at left, enolate at right, showing movement of electron pairs resulting in deprotonation of the stable parent enediol. A distinct, more complex chemical system, exhibiting the characteristic of vinylogy.]]

Ribulose-1,5-bisphosphate is a key substrate in the Calvin cycle of photosynthesis. In the Calvin cycle, the ribulose equilibrates with the enediol, which then binds carbon dioxide.{{cn|date=February 2024}} The same enediol is also susceptible to attack by oxygen (O<sub>2</sub>) in the (undesirable) process called photorespiration. :[[File:EnediolPhotoResp.svg|thumb|center|class=skin-invert-image|Keto-enediol equilibrium for ribulose-1,5-bisphosphate.]]

== See also == * Alkenal * Enolase * Geminal diol, another form of ketones and aldehydes in water solutions * Ketone * Regioselectivity * Ynol

== References == {{reflist}}

==External links== {{wikiquote}} * [http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_13%3a_Reactions_with_stabilized_carbanion_intermediates_I/Section_13.1%3a_Tautomers Enols and enolates in biological reactions] {{Webarchive|url=https://web.archive.org/web/20120313041632/http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter_13%3a_Reactions_with_stabilized_carbanion_intermediates_I/Section_13.1%3a_Tautomers |date=2012-03-13 }}

{{Authority control}}

{{DEFAULTSORT:Keto-Enol Tautomerism}} Category:Functional groups Category:Metabolism Category:Reactive intermediates Category:Alcohols Category:Alkene derivatives Category:Enols Category:Organic reactions