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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.
The keto and enol tautomers of acetylacetone coexist in solution. The enol form has C 2v symmetry, meaning the hydrogen atom is shared equally between the two oxygen atoms. [4] In the gas phase, the equilibrium constant, K keto→enol, is 11.7, favoring the enol form.
Most common tautomers exist in pairs, which means that the hydrogen is located at one of two positions, and even more specifically the most common form involves a hydrogen changing places with a double bond: H−X−Y=Z ⇌ X=Y−Z−H. Common tautomeric pairs include: [3] [4] ketone – enol: H−O−C=C ⇌ O=C−C−H, see keto–enol ...
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]
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.
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 ...