Enol

EnolizationEdit

Stereochemistry of ketonizationEdit

EnediolsEdit

PhenolsEdit

BiochemistryEdit

ReactivityEdit

Stable enolsEdit

Addition of electrophilesEdit

Deprotonation: enolatesEdit

In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula Template:Chem2 (R = many substituents). The term enol is an abbreviation of alkenol, a portmanteau deriving from "-ene"/"alkene" and the "-ol". Many kinds of enols are known.<ref name=March/>

Keto–enol tautomerism refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. The keto and enol forms are tautomers of each other.<ref name="Clayden-2012">Template:Cite book</ref>

Organic esters, ketones, and aldehydes with an α-hydrogen (Template:Chem2 bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton (H) from carbon to oxygen:<ref name=March>Template:Cite book</ref>

Template:Chem2

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>Template:Cite journal</ref>

File:AcacH.svg

Template:Clear-left

Selected enolization constants<ref>Template:Cite journal</ref>
carbonyl	enol	K_enolization
Acetaldehyde Template:Chem2	Template:Chem2	5.8Template:X10^
Acetone Template:Chem2	Template:Chem2	5.12Template:X10^
Methyl acetate Template:Chem2	Template:Chem2	4Template:X10^
Acetophenone Template:Chem2	Template:Chem2	1Template:X10^
Acetylacetone Template:Chem2	Template:Chem2	0.27
Trifluoroacetylacetone Template:Chem2	Template:Chem2	32
Hexafluoroacetylacetone Template:Chem2	Template:Chem2	~10⁴
Cyclohexa-2,4-dienone	Phenol Template:Chem2	>10¹²

Enols are derivatives of vinyl alcohol, with a Template:Chem2 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 = [enol]/[keto] ≈ 3Template:X10^). In 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.

If R¹ and R² (note equation at top of page) are different substituents, there is a new stereocenter formed at the alpha position when an enol converts to its keto form. Depending on the nature of the three R groups, the resulting products in this situation would be diastereomers or enantiomers.Template:Cn

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>Template:Cite journal</ref>

File:Keto-Endiol-Tautomerie.svg

Keto-enediol tautomerizations. Enediol in the center; acyloin isomers at left and right. Ex. is hydroxyacetone, shown at right.

File:Ascorbic acidity3.png

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. The same enediol is also susceptible to attack by oxygen (O₂) in the (undesirable) process called photorespiration.

File:EnediolPhotoResp.svg

Keto-enediol equilibrium for ribulose-1,5-bisphosphate.

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 involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.<ref>Template:Cite journal</ref>

File:Tetrahydronaphthalenedione.png

Keto–enol tautomerism is important in several areas of biochemistry.Template:Cn

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.Template:Cn

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>Template:Cite book</ref>

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The terminus of the double bond in enols is nucleophilic. Its reactions with electrophilic organic compounds is important in biochemistry as well as synthetic organic chemistry. In the former area, the fixation of carbon dioxide involves addition of CO₂ to an enol.Template:Cn

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Deprotonation of enolizable ketones, aldehydes, and esters gives enolates.<ref>Template:March6th</ref><ref name=enolate>Template:Cite book</ref> Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether.<ref>Mukaiyama, T.; Kobayashi, S. Org. React. 1994, 46, 1. {{#invoke:doi|main}}</ref> Acylation gives esters such as vinyl acetate.<ref name=Ullmann>Template:Cite encyclopedia</ref>

File:2-phospho-D-glycerate wpmp.png

File:Phosphoenolpyruvate wpmp.png

File:Pyruvate wpmp.png

File:Biochem reaction arrow reversible NYYN horiz med.svg

File:Biochem reaction arrow reversible YYNN horiz med.svg

In general, enols are less stable than their keto equivalents because of the favorability of the C=O double bond over C=C double bond. However, enols can be stabilized kinetically or thermodynamically.Template:Cn

Some enols are sufficiently stabilized kinetically so that they can be characterized.Template:Cn

File:Hindrance.png

Diaryl-substitution stabilizes some enols.<ref>Template:Cite journal</ref>

Delocalization can stabilize the enol tautomer. Thus, very stable enols are phenols.<ref name=":0">Template:Cite book</ref> Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding.<ref>Template:Cite journal</ref> Both of these factors influence the enol-dione equilibrium in acetylacetone.