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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]
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.
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.
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 ...
Some kinds of molecules could have different minimum-energy tautomers in different electronic states, and if the molecular structure of minimum-energy tautomer in the excited state is proton-transferred geometry between neighboring atoms, proton transfer in excited state can occur. The tautomerization often takes the form of keto-enol tautomerism.
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]
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.
Common reaction conditions include use of sodium or potassium alkoxide as base in hexane or tetrahydrofuran solvents, at temperatures ranging between 200–400 °C. A hydrolysis step is also required in the synthesis. The Madelung synthesis is important because it is one of few known reactions that produce indoles from a base-catalyzed thermal ...