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In organic chemistry, keto–enol tautomerism refers to a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (an alcohol). The enol and keto forms are said to be tautomers of each other. The interconversion of the two forms involves the movement of an alpha hydrogen and the shifting of bonding electrons; hence, the isomerism qualifies as tautomerism. A compound containing a carbonyl group (C=O) is normally in rapid equilibrium with an enol tautomer, which contains a pair of doubly bonded carbon atoms adjacent to a hydroxyl (−OH) group, C=C-OH. The keto form predominates at equilibrium for most ketones. Nonetheless, the enol form is important for some reactions. The deprotonated intermediate in the interconversion of the two forms, referred to as an enolate anion, is important in carbonyl chemistry, in large part because it is a strong nucleophile. Normally, the keto–enol tautomerization chemical equilibrium is highly thermodynamically driven, and at room temperature the equilibrium heavily favors the formation of the keto form. A classic example for favoring the keto form can be seen in the equilibrium between vinyl alcohol and acetaldehyde (K = ()/() ≈ 3 × 10−7). However, it is reported that in the case of vinyl alcohol, formation of a stabilized enol form can be accomplished by controlling the water concentration in the system and utilizing the kinetic favorability of the deuterium produced kinetic isotope effect (''k''H+/''k''D+ = 4.75, ''k''H2O/''k''D2O = 12). Deuterium stabilization can be accomplished through hydrolysis of a ketene precursor in the presence of a slight stoichiometric excess of heavy water (D2O). Studies show that the tautomerization process is significantly inhibited at ambient temperatures ( ''k''t ≈ 10−6 M/s), and the half life of the enol form can easily be increased to ''t''1/2 = 42 minutes for first order hydrolysis kinetics.〔'' Investigations into the Chemistry of Thermodynamically Unstable Species. The Direct Polymerization of Vinyl Alcohol, the Enolic Tautomer of Acetaldehyde.'' Anna K. Cederstav and Bruce M. Novak. Journal of the American Chemical Society, 1994, Volume 116, Pages 4073–74. ()〕 ==Mechanism== The acid catalyzed conversion of an enol to the keto form proceeds by a two step mechanism in an aqueous acidic solution. For this, it is necessary that the alpha carbon (the carbon closest to functional group) contains at least one hydrogen atom known as alpha hydrogen.This atom is removed from the alpha carbon and bonds to the oxygen of the carbonyl carbon to form the enol tautomer. The existence of hydrogen atom at alpha carbon is necessary but not sufficient condition for enolization to occur. To be acidic, the alpha hydrogen should be positioned such that may line up parallel with antibonding pi-orbital of the carbonyl group. The hyperconjugation of this bond with C–H bond at alpha carbon reduces the electron density out of C–H bond and weakens it. Thus the alpha hydrogen becomes acidic. When this requirement is not enforced, for example in the adamantanone or other polycyclic ketones, the enolization is impossible or very slow. (J. E. Ordlander et al., Resistance of Adamantanone to Homoenolization, 1969), (J.B. Stothers and C.T. Tan,Adamantanone: Stereochemistry of its Homoenolization as shown by 2H Nuclear Magnetic Resonance, 1974) First, the exposed electrons of the C=C double bond of the enol are donated to a hydronium ion (H3O+). This addition follows Markovnikov's rule, thus the proton is added to the carbon with more hydrogens. This is a concerted step with the oxygen in the hydroxyl group donating electrons to produce the eventual carbonyl group. File:Keto-enol.svg 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Keto–enol tautomerism」の詳細全文を読む スポンサード リンク
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