Enolate interconversion

Simplifications ease the extraction of accurate values for the rate constants. For example, the keto, enol interconversion may sometimes be ignored, and k 2 > Xc, and Xtj > k are justifiable assumptions. ... 

The enol tautomers of many ketones and aldehydes, carboxylic acids, esters and amides, ketenes, as well as the keto tautomers of phenols have since all been generated by flash photolysis to determine the pH rate profiles for keto-enol interconversion. Equilibrium constants of enolization, KB, were determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK, Equation (1). 

We note that for the case of keto/enol interconversion, it is simple, useful and remarkably accurate to equate the difference of Gibbs energies in aqueous media of the two tautomers and the difference of enthalpies of formation in the gas J. R. Keefe and A. J. Kresge, J. Phys. Org. Chem., 5, 575 (1992). We are pleased. 

Considerations of entropies of activation played a major role in a recent discussion of the acid-catalyzed interconversion of 1,2-cyclo-hexanedione and its enol (Bakule and Long, to be published). The reaction is more complex than the usual keto-enol interconversion because the ketone form, but not the enol, is found to be hydrated. 

The structures of most benzophenones have been determined by 1D ( H, l3C, and DEPT) and 2D (COSY, HSQC, HMBC, and NOESY) NMR experiments. The majority of benzophenone NMR spectra have been recorded in CDCI3 and CD3OD. Benzene-c4, mixtures of benzene-<4 and CDCI3 [75], or pyridine-ds have also been used [88,93]. The aforementioned solvents were used to resolve overlapping signals of studied compounds. Deuterated TFA (0.1%) has also been used to increase the rate of keto-enol interconversion in benzophenones. We now turn our attention to the structural elucidation of xanthochymol (138). 

Now that we know that a hydrogen on a carbon adjacent to a carbonyl carbon is somewhat acidic, we can understand why keto and enol tautomers interconvert as we first saw in Chapter 6. Keto-enol interconversion is also called keto-enol tautomer-ization or enolization. The interconversion of the tautomers can be catalyzed by either acids or bases. 

Notice the similarity between keto-enol interconversion and a-substitution. Actually, keto-enol interconversion is an a-substitution reaction in which hydrogen serves as both the electrophile that is removed from the a-carbon and the electrophile that is added to the a-carbon (when the enol or enolate reverts back to the keto tautomer). 

Keto-enol interconversion can be catalyzed by acids or by bases. Generally, the keto tautomer is more stable. When an a-substitution reaction takes place under acidic conditions, an enol reacts with an electrophile when the reaction takes place under basic conditions, an enolate ion reacts with an electrophile. Whether C or O reacts with the electrophile depends on the electrophile and the reaction conditions. 

Hunsdiecker reaction (p. 820) a-hydrogen (p. 788) keto-enol interconversion (p. 792)... 

Both an acidic and an alkaline environment will catalyze a shift of tiie equilibrium to the right, making the portion of the molecule actually present in the enol form higher. This equilibrium runs through a number of steps that are different depending on whether the solution is acid or alkaline. If the solution is alkaline, the keto-enol interconversion occurs via the enolate ion. In this enolate ion a proton is abstracted from either the alcohol group or the a-carbon of the enol form, and the two forms are stabilized by resonance (Figure 3.17.3). 

Any reaction that simply involves the intramolecular transfer of a proton is called a tautomerism. Keto-enol interconversion may happen in basic as well as acidic solution. The steps are reversed in the base-catalyzed and acid-catalyzed reactions. In the base-catalyzed reaction, the first step is removal of an a-proton and the... 

A study of the thermodynamics and pH-dependent kinetics of keto-enol/enolate interconversion of A-methylindoline-2-one and its 2-thione and 2-selone analogues in aqueous solution has allowed the estimation of the respective values for the carbon acid ionization constant (Qf), enol acidity constant (Qf) and keto-enol equilibrium constant (A e)- ... 

A ketone and an enol differ only in the location of a double bond and a hydrogen. A ketone and its corresponding enol are called keto-enol tautomers. Tautomers ( taw-toe-mers ) are constitutional isomers that are in rapid equilibrium. The keto tautomer predominates in solution, because it is usually much more stable than the enol tautomer. Interconversion of the tautomers is called keto-enol interconversion or tautomerization. 

MECHANISM FOR ACID-CATALYZED KETO-ENOL INTERCONVERSION... 

Interconversion of the tautomers is called tautomerization or keto-enol interconversion. 

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