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Keto–enol tautomerism refers to a chemical equilibrium between a "keto" form (a carbonyl, named for the common ketone case) and an enol. The interconversion of the two forms involves the transfer of an alpha hydrogen atom and the reorganisation of bonding electrons. The keto and enol forms are tautomers of each other. [2]
Edens et al. have investigated the reaction mechanism. [6] They found it was characterized by three major steps: (1) the rapid protonation of oxygen, (2) the slow, rate-determining step comprising the 1,3-shift of the protonated hydroxy group, and (3) the keto-enol tautomerism followed by rapid deprotonation.
Many carbonyl compounds exhibit keto–enol tautomerism. This effect is especially pronounced in 1,3-dicarbonyl compounds that can form hydrogen-bonded enols. The equilibrium constant is dependent upon the solvent polarity, with the cis -enol form predominating at low polarity and the diketo form predominating at high polarity.
Some examples of tautomers Keto-enol tautomerization typically strongly favors the keto tautomer, but an important exception is the case of 1,3-diketones such as acetylacetone. [6] Tautomerization is pervasive in organic chemistry.
The Conrad-Limpach reaction mechanism also involves multiple keto-enol tautomerizations, all of which are catalyzed through the use of a strong acid, often HCl or H 2 SO 4. With much of the literature on the synthesis of quinolines , there is some discrepancy on whether a substituted 4-hydroxyquinoline or a substituted 4-quinolone is the final ...
The equilibrium constant tends to be high in nonpolar solvents; when K keto→enol is equal or greater than 1, the enol form is favoured. The keto form becomes more favourable in polar, hydrogen-bonding solvents, such as water. [7] The enol form is a vinylogous analogue of a carboxylic acid. [citation needed]
Keto-enol tautomerism. 1 is the keto form; 2 is the enol. Ketones that have at least one alpha-hydrogen, undergo keto-enol tautomerization; the tautomer is an enol. Tautomerization is catalyzed by both acids and bases. Usually, the keto form is more stable than the enol. This equilibrium allows ketones to be prepared via the hydration of alkynes.
The tautomerization can also be catalyzed via photochemical process. These findings suggest that the keto–enol tautomerization is a viable route under atmospheric and stratospheric conditions, relevant to a role for vinyl alcohol in the production of organic acids in the atmosphere. [5] [6]