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Enol
Enols, or more formally, alkenols, are a type of reactive structure or intermediate in organic chemistry that is represented as an alkene (olefin) with a hydroxyl group attached to one end of the alkene double bond. The terms enol and alkenol are portmanteaus deriving from "-ene"/"alkene" and the "-ol" suffix indicating the hydroxyl group of alcohols, dropping the terminal "-e" of the first term. Generation of enols often involves removal of a hydrogen adjacent (α-) to the carbonyl group—i.e., deprotonation, its removal as a proton, H+. When this proton is not returned at the end of the stepwise process, the result is an anion termed an enolate (see images at right). The enolate structures shown are schematic; a more modern representation considers the molecular orbitals that are formed and occupied by electrons in the enolate. Similarly, generation of the enol often is accompanied by "trapping" or masking of the hydroxy group as an ether, such as a silyl enol ether.〔Douglass F. Taber, 2015, "Synthesis of silyl enol ethers and related compounds," ''Organic Chemistry Portal'' (online), Reactions, Organic Synthesis Search, Categories, O-Si Bond Formation, see (), accessed 16 July 2015.〕
Critically, the terminus of the double bond in enols, enolates, and silyl enol ethers that lacks the hydroxyl group is nucleophilic, and so can act as an electron donor in reactions with electrophilic organic compounds; this reactivity underlies the tremendous importance of enol-based intermediates in a wide array of important life processes (i.e., in biochemistry, as intermediates in enzyme-catalysed reactions), as well as being central to modern synthetic organic chemistry (e.g., in applications of aldol and related reactions). The importance of enols in accomplishing nature and humankind's chemical transformations makes them irreplaceable.
The process to form enols and related structures requires at least one hydrogen atom on a carbon adjacent to a carbonyl group, a requirement that is met in various aldehyde, ketone, ester, and other more complicated organic compounds, including many carbocycles and heterocycles.The process by which they form, keto-enol tautomerization, is an isomerization of a parent organic compounds, involving interconversion via formal migration of a hydrogen atom accompanied by switch of adjacent single and double bonds, in this case, the hydrogen atom in the position immediately adjacent (α-) to the carbonyl group, as was shown and discussed above.
The types and sizes of the substituents in the vicinity of the enol and enolate (e.g., R1, R2, and R3 in the image above), and the particulars of reaction solvent, additives, and temperature contribute to the stability of the enol-type isomer, and fix the point at which an equilibrium lies, and for enols, the ability of the enol's hydroxyl group to hydrogen bond with an adjacent group can make it the preponderant isomer in solution (e.g., 3:1 and more over the keto form). Moreover, the substituents and conditions determine the preponderant conformations of these reactive species, and therefore dictate the stereochemical outcomes of their reactions. In biochemical transformations, the control of such reactions has been understood as exquisite for as long as the reactions have been studied, and in modern chemical syntheses the same control is increasingly being achieved by man. Likewise, more complex substituents contribute to further unique reactivities, such as in enediols, alkenes with two hydroxyl groups, one on each end of the double bond, which appear in such structures as that of vitamin C, and contribute to its important redox properties.
== Keto-enol tautomerism ==
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