Keto-enol Tautomerism

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. Furthermore, 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 x10−7). 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 (kH+/kD+ = 4.75, kH2O/kD2O = 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 ( kt ≈ 10−6 M/s), and the half life of the enol form can easily be increased to t1/2 = 42 minutes for first order hydrolysis kinetics.

Read more about Keto-enol Tautomerism:  Mechanism, Erlenmeyer Rule, Stereochemistry of Ketonization, Phenols, Significance in Biochemistry

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... Thus, at equilibrium, over 99% of propanedial (OHCCH2CHO) molecules exist as the mono-enol ... The percentage is lower for 1,3-aldehyde ketones and diketones (acetylacetone, for example, 80% enol form) ...