View source for Enol

{{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|3-Pentanone]], a less stabilized enol (ketone left, enol right)
 | image2 = Enolate Resonance.svg
 | alt2 = TBD
 | caption2  = [[2-butanone|2-Butanoate]] resonance forms ([[carbanion]] left, [[enolate|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 [[chemical intermediate|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 [[Tautomer|tautomeric]] interconversion involves hydrogen atom movement and the reorganisation of bonding [[electron]]s.<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 anion|enolate anions]], which are important in organic reaction strategies as a strong [[nucleophile]]. 

==Enolization==
[[organic compound|Organic]] [[ester]]s, [[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 [[Acetyl group|methyl ketone]]), then it is [[Prochirality|prochiral]], forming a new stereocenter when in keto form.  Conversely, enolization [[Racemization|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 [[nucleophile|nucleophilic]], a property enhanced in the case of [[Enolate anion|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}}</ref>  As many organic syntheses involve the controlled formation and reaction of enolates, enols appear transiently in great quantities during [[Quenching (chemistry)|quenching]].<ref name=":1" /><ref name="enolate" />   

=== Stable enols ===
[[File:Hindrance.png|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 [[phenol]]s.<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=[[Journal of Chemical Education|J. Chem. Educ.]]|volume=88|issue=10|pages=1444–1445|doi=10.1021/ed1010932
|bibcode=2011JChEd..88.1444M}}</ref>
:[[File:AcacH.svg|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 logarithm]]s 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
|-
|[[1,1,1-Trifluoroacetylacetone|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 ====
[[Phenol]]s represent a kind of enol.  For some phenols and related compounds, the keto tautomer plays an important role.  Many of the reactions of [[resorcinol]] involve the keto tautomer, 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>
:[[File:Tetrahydronaphthalenedione.svg|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" | [[image:2-phospho-D-glycerate wpmp.png|class=skin-invert-image]]
| colspan="2" style="width:75px" |
| rowspan="5" | [[image:phosphoenolpyruvate wpmp.png|class=skin-invert-image]]
| colspan="2" style="width:75px" |
| rowspan="5" | [[image:pyruvate wpmp.png|class=skin-invert-image]]
|-
|
| H<sub>2</sub>O
| [[Adenosine diphosphate|ADP]]
| [[adenosine triphosphate|ATP]]
|-
| colspan="2" style="width:75px" | [[image:Biochem reaction arrow reversible NYYN horiz med.svg|75px|class=skin-invert-image]]
| colspan="2" style="width:75px" | [[image:Biochem reaction arrow reversible YYNN horiz med.svg|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 [[acyloin]]s.  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 [[reductone]]s.  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; [[acyloin]]s 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]]
* [[Ketone]]
* [[Ynol]]
* [[Geminal diol]], another form of ketones and aldehydes in water solutions
* [[Regioselectivity]]

== 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]

{{Authority control}}

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