Enol

In organic chemistry, enols are a type of functional group or intermediate in organic chemistry containing a group with the formula C=C(OH) (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.^[1]

Examples of keto-enol tautomerism

Ketone tautomerization, keto-form at left, enol at right. Ex. is 3-pentanone, a less stabilized enol.^{[citation needed]}

Enolate resonance structures, schematic representation of forms (see text regarding molecular orbitals); carbanion form at left, enolate at right; Ex. is 2-butanone, also a less stabilized enol.^{[citation needed]}

Ketone tautomerization, enol-form at left, keto at right. Ex. is 2,4-pentanedione, a hydrogen bond (---) stabilized enol.^{[citation needed]}

Aldehyde tautomerization, enol-form at left, "keto" at right; Ex. is tartronaldehyde (reductone), an enediol-type of enol.^{[citation needed]}

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.^[2]

Enolization

Organic esters, ketones, and aldehydes with an α-hydrogen (C−H bond adjacent to the carbonyl group) often form enols. The reaction involves migration of a proton (H) from carbon to oxygen:^[1]

RC(=O)CHR′R′′ ⇌ RC(OH)=CR′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:^[3]

Selected enolization constants^[4]

carbonyl	enol	Kenolization
Acetaldehyde CH3CHO	CH2=CHOH	5.8×10−7
Acetone CH3C(O)CH3	CH3C(OH)=CH2	5.12×10−7
Methyl acetate CH3CO2CH3	CH2=CH(OH)OCH3	4×10−20
Acetophenone C6H5C(O)CH3	C6H5C(OH)=CH2	1×10−8
Acetylacetone CH3C(O)CH2C(O)CH3	CH3C(O)CH=C(OH)CH3	0.27
Trifluoroacetylacetone CH3C(O)CH2C(O)CF3	CH3C(O)CH=C(OH)CF3	32
Hexafluoroacetylacetone CF3C(O)CH2C(O)CF3	CF3C(O)CH=C(OH)CF3	~104
Cyclohexa-2,4-dienone	Phenol C6H5OH	>1012

Enols are derivatives of vinyl alcohol, with a C=C−OH 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] ≈ 3×10⁻⁷). 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.

Stereochemistry of ketonization

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.^{[citation needed]}

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.^[5]

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

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.

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

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 involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.^[6]

Biochemistry

Keto–enol tautomerism is important in several areas of biochemistry.^{[citation needed]}

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.^{[citation needed]}

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.^[7]

		H2O		ADP	ATP
		H2O

Reactivity

See also: Carbonyl α-substitution reactions

Addition of electrophiles

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.^{[citation needed]}

Deprotonation: enolates

Main article: enolate

Deprotonation of enolizable ketones, aldehydes, and esters gives enolates.^[8][9] Enolates can be trapped by the addition of electrophiles at oxygen. Silylation gives silyl enol ether.^[10] Acylation gives esters such as vinyl acetate.^[11]

Stable enols

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.^{[citation needed]}

Some enols are sufficiently stabilized kinetically so that they can be characterized.^{[citation needed]}

Diaryl-substitution stabilizes some enols.^[12]

Delocalization can stabilize the enol tautomer. Thus, very stable enols are phenols.^[13] Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding.^[14] Both of these factors influence the enol-dione equilibrium in acetylacetone.

See also

• Alkenal
• Enolase
• Ketone
• Ynol
• Geminal diol, another form of ketones and aldehydes in water solutions
• Regioselectivity