In organic chemistry, keto-enol tautomerism is a fundamental concept describing the equilibrium between two forms of a molecule: the "keto" form (a carbonyl compound) and the "enol" form (an alkene with a hydroxyl group attached to one of the alkene carbons). This interconversion involves the migration of an alpha-hydrogen atom and the rearrangement of bonding electrons. The term "enol" is a portmanteau of "alkene" and "alcohol," reflecting its structural features.
The Basics of Keto-Enol Tautomerism
Keto-enol tautomerism is a chemical equilibrium between a "keto" form and an enol. Organic esters, ketones, and aldehydes with an α-hydrogen (C−H bond adjacent to the carbonyl group) often form enols. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. 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. 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.
Defining Enols and Keto Forms
- Enol: An enol is a functional group or intermediate in organic chemistry containing a group with the formula C=C(OH) (R = many substituents). Enols are derivatives of vinyl alcohol, featuring a carbon-carbon double bond adjacent to a hydroxyl group (C=C-OH).
- Keto: The keto form is a carbonyl, named for the common ketone case. It contains a carbon-oxygen double bond (C=O).
The Tautomerization Process
The conversion between keto and enol forms involves the movement of a proton from the alpha carbon (the carbon atom next to the carbonyl group) to the oxygen atom of the carbonyl group. This is accompanied by a shift of the double bond from between the carbon and oxygen to between the alpha and beta carbons.
Factors Influencing Keto-Enol Equilibrium
While keto-enol tautomerism exists as an equilibrium, the position of this equilibrium can be influenced by several factors.
Stability of the Isomers
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. The keto form is usually strongly favored due to the greater stability of the C=O bond compared to the C=C bond. 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).
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Delocalization can stabilize the enol tautomer. In some compounds with two (or more) carbonyls, the enol form becomes dominant.
Substituent Effects
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.
Solvent Effects
The solvent can also influence the equilibrium. Polar protic solvents tend to favor the enol form, while nonpolar solvents favor the keto form.
Enediols and Reductones
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. Keto-enediol tautomerizations. Enediol in the center; acyloin isomers at left and right. Ex. 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.
Enediols
Enediols are a special class of enols featuring a hydroxyl group on each carbon of the C=C double bond. While generally less stable than their corresponding acyloin isomers, enediols can be stabilized in certain situations.
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Reductones
Reductones are enediols stabilized by flanking carbonyl groups.
Keto-Enol Tautomerism in Sugars
In sugars, the linear and cyclic hemiketal or hemiacetal of the sugar exist in equilibrium; in the linear form, sugars can undergo keto-enol tautomerism. This takes place during the interconversion between the aldose and ketose forms.
Enolates
The conjugate base of the enediol, called an enolate may also serve as an intermediate for another reaction called epimerisation. The resulting deprotonated C2 oxygen donates a lone electron pair to the C2 carbon and thus form a carbonyl at this position.
Importance in Sugar Biochemistry
Keto-enol tautomerism is an important process in sugar biochemistry. The interconversion of an aldose to a ketose, such as D- glucose to D-fructose, occurs via their common enolate isomer. This is also true of epimerisation reactions, that allow interconversion of two aldoses, such as D-glucose and D-mannose or two ketoses, such as D-psicose and D-fructose.
Aldoses and Ketoses
To convert between a 6-membered aldose, the sugar must tautomerise, to give an intermediate called and ene-diol, so-called because there is an alcohol group adjacent to the carbonyl (diol, two alcohols). The base removes the proton adjacent to the anomeric, carbonyl carbon. This is referred to as the alpha hydrogen. In doing so, a double bond between the alpha carbon and carbonyl carbon is formed, as one of the C=O carbonyl bonds break, and the liberated electron pair on oxygen picks up a proton from an acid. This amounts to the movement of a proton from the alpha position to the carbonyl oxygen. One bond of the C=C bond opens to accommodate a hydrogen, provided by a base. The reaction is reversible; the proton is transferred from the C1 OH group along the C=C bond to the C2 alpha position.
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Phenols and Keto Tautomers
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.
Biochemical Significance
The enzyme enolase catalyzes the dehydration of 2-phosphoglyceric acid to the enol phosphate ester. 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.
Reactivity of Enols
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. Deprotonation of enolizable ketones, aldehydes, and esters gives enolates.[8][9] Enolates can be trapped by the addition of electrophiles at oxygen.
Nucleophilicity
The terminus of the double bond in enols is nucleophilic.
Reactions with Electrophiles
Its reactions with electrophilic organic compounds is important in biochemistry as well as synthetic organic chemistry.
Enolates
Deprotonation of enolizable ketones, aldehydes, and esters gives enolates. Enolates can be trapped by the addition of electrophiles at oxygen.
Acid-Catalyzed Conversion
The acid-catalyzed conversion of an enol to the keto form proceeds by proton transfer from O to carbon.
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