Inorganic chemistry,enols are a type offunctional group orintermediate inorganic chemistry. Formally, enols are derivatives ofvinyl alcohol, with aC=C−OH connectivity. The termenol is an abbreviation ofalkenol, aportmanteau deriving from "-ene"/"alkene" and "-ol"/"alcohol".
Keto–enol tautomerism refers to achemical equilibrium between a "keto" form (acarbonyl, named for the commonketone case) and an enol. Thetautomeric interconversion involves hydrogen atom movement and the reorganisation of bondingelectrons.[1]
Many kinds of enols are known, but very few are stable compounds.[2] However, deprotonation of organic carbonyls givesenolate anions, which are important in organic reaction strategies as a strongnucleophile.
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:[2]
The process does not occur intramolecularly, but requires participation of solvent or other mediators.[citation needed]
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 adouble bond. If the α position of an enol is substituted (i.e., not amethyl ketone), then it isprochiral, forming a new stereocenter when in keto form. Conversely, enolizationracemizes that stereocenter.[citation needed]
Usually the tautomerization equilibrium constant is so small that the enol is undetectable spectroscopically. In the equilibrium betweenvinyl alcohol andacetaldehyde,K = [enol]/[keto] ≈ 5.8×10−7.[3]
The terminus of the double bond in enols isnucleophilic, a property enhanced in the case ofenolate anions.[4][5] However, enolates protonate reversibly at the oxygen much faster than equilibrate to the ketone/aldehyde/etc.[6] As many organic syntheses involve the controlled formation and reaction of enolates, enols appear transiently in great quantities duringquenching.[4][5]
Enols can be stabilized throughvinylogy. Thus, very stable enols arephenols.[8]
In compounds with two (or more) carbonyls, the enol form is also stabilized through intramolecular hydrogen bonding[9] and becomes dominant. The behavior of2,4-pentanedione illustrates this effect:[10]
| 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 |
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.[11]
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.[12]
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| H2O | ADP | ATP | ||||
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| H2O | ||||||
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.[13]
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.[citation needed] The same enediol is also susceptible to attack by oxygen (O2) in the (undesirable) process calledphotorespiration.
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