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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.
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 .
Keto enol tautomerization (diketo form on left, cis-enol form on right) Many carbonyl compounds exhibit keto–enol tautomerism. This effect is especially pronounced in 1,3-dicarbonyl compounds that can form hydrogen-bonded enols.
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.
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.
4-Pyridone exists a keto-enol tautomerism with its enol tautomer 4-hydroxypyridine. In solution, the keto tautomer is favoured, [4] and the enol tautomer only becomes important in very dilute solutions or solutions of non-polar solvents. However, the enol tautomer is dominant in the gas phase. [5]
The enamine-imine tautomerism may be considered analogous to the keto-enol tautomerism. In both cases, a hydrogen atom switches its location between the heteroatom (oxygen or nitrogen) and the second carbon atom. Enamines are both good nucleophiles and good bases.
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]