Editing Enol (section)

==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′′ }}
In the case of ketones, the conversion is called a keto-enol tautomerism, although this name is often more generally applied to all such tautomerizations.  Usually the equilibrium constant is so small that the enol is undetectable spectroscopically.

In some compounds with two (or more) carbonyls, the enol form 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>{{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}}</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>
|}

Enols are derivatives of [[vinyl alcohol]], with a {{chem2|C\dC\sOH}} connectivity. Deprotonation of organic carbonyls gives the [[enolate anion]], which are a strong [[nucleophile]]. A classic example for favoring the keto form can be seen in the equilibrium between [[vinyl alcohol]] and [[acetaldehyde]] (K&nbsp;=&nbsp;[enol]/[keto]&nbsp;≈&nbsp;3{{x10^|-7}}). In [[diketone#1,3-Diketones|1,3-diketones]], such as [[acetylacetone]] (2,4-pentanedione), the enol form is more favored.

The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon.  The process does not occur intramolecularly, but requires participation of solvent or other mediators.