Inorganic chemistry,enols are a type offunctional group orintermediate inorganic chemistry containing a group with the formulaC=C(OH) (R = many substituents). The termenol is an abbreviation ofalkenol, aportmanteau deriving from "-ene"/"alkene" and the "-ol". Many kinds of enols are known.[1]
Keto–enol tautomerism refers to achemical equilibrium between a "keto" form (acarbonyl, named for the commonketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bondingelectrons. The keto and enol forms aretautomers of each other.[2]
Organicesters,ketones, andaldehydes with anα-hydrogen (C−H bond adjacent to thecarbonyl group) often form enols. The reaction involves migration of a proton (H) from carbon to oxygen:[1]
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 of2,4-pentanedione illustrates this effect:[3]
| 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 ofvinyl alcohol, with aC=C−OH connectivity. Deprotonation of organic carbonyls gives theenolate anion, which are a strongnucleophile. A classic example for favoring the keto form can be seen in the equilibrium betweenvinyl alcohol andacetaldehyde (K = [enol]/[keto] ≈ 3×10−7). In1,3-diketones, such asacetylacetone (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 bediastereomers orenantiomers.[citation needed]
Enediols are alkenes with a hydroxyl group on each carbon of the C=C double bond. Normally such compounds are disfavored components in equilibria withacyloins. One special case iscatechol, 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 calledreductones. Such species are important in glycochemistry, e.g., theLobry de Bruyn–Van Ekenstein transformation.[5]
Ribulose-1,5-bisphosphate is a key substrate in theCalvin cycle ofphotosynthesis. In the Calvin cycle, the ribulose equilibrates with the enediol, which then bindscarbon dioxide. The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process calledphotorespiration.
Phenols represent a kind of enol. For some phenols and related compounds, the keto tautomer plays an important role. Many of the reactions ofresorcinol involve the keto tautomer, for example. Naphthalene-1,4-diol exists in observable equilibrium with the diketone tetrahydronaphthalene-1,4-dione.[6]
Keto–enol tautomerism is important in several areas ofbiochemistry.[citation needed]
The high phosphate-transfer potential ofphosphoenolpyruvate 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]
Theenzymeenolase catalyzes the dehydration of2-phosphoglyceric acid to the enol phosphate ester. Metabolism of PEP topyruvic acid bypyruvate kinase (PK) generatesadenosine triphosphate (ATP) viasubstrate-level phosphorylation.[7]
 |  |  | ||||
| H2O | ADP | ATP | ||||
 |  | |||||
| H2O | ||||||
The terminus of the double bond in enols isnucleophilic. Its reactions withelectrophilic organic compounds is important inbiochemistry as well assynthetic organic chemistry. In the former area, the fixation of carbon dioxide involves addition of CO2 to an enol.[citation needed]
Deprotonation of enolizable ketones, aldehydes, and esters givesenolates.[8][9] Enolates can be trapped by the addition of electrophiles at oxygen. Silylation givessilyl enol ether.[10] Acylation givesesters such asvinyl acetate.[11]
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]
Delocalization can stabilize the enol tautomer. Thus, very stable enols arephenols.[13] Another stabilizing factor in 1,3-dicarbonyls is intramolecular hydrogen bonding.[14] Both of these factors influence the enol-dione equilibrium in acetylacetone.
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