Equilibrium constants for enolization

The amount of enol present at equilibrium the enol content is quite small for sim pie aldehydes and ketones The equilibrium constants for enolization as shown by the following examples are much less than 1... 

Finally, concordant results have been obtained from a kinetic study of the iodination of acetophenone and acetone at very low iodine concentration (Verny-Doussin, 1979). The procedure used is similar to that followed for the determination of equilibrium constants for enol formation by the kinetic-halogenation method, i.e. second-order rate constants were measured under conditions such that halogen additions to enol and enolate are rate-limiting (43). Under these conditions, the experimental kn-values can be expressed by... 

It is of great interest to compare this last value with the keto-enol equilibrium constant obtained similarly for acetone = 0.35 x 10-8). Indeed, in many enzyme-catalysed reactions, aldolisation for example, enamine formation is not rate-limiting, and the rate is usually controlled by subsequent electrophilic additions. Consequently, the rate depends on enamine reactivity and on the enamine concentration at equilibrium. Therefore, if one wants to compare the two processes, via enol and via enamine, in order to explain why the enamine route is usually preferred, the difference in equilibrium constants for enol and enamine formation must be taken into account. Data on ketone to enol and ketone to enamine equilibrium constants show that the enamine and enol concentrations are of similar magnitude even for relatively small concentrations of primary amine. Thereafter, since the enamine is much more reactive than the enol for reactions with electrophilic reagents (in a ratio of 4-6 powers of ten for proton addition), it can be easily understood why the amine-catalysed pathway is energetically more favourable. 

Our experience to this point has been that C—H bonds are not very acidic. Compared with most hydrocarbons, however, aldehydes and ketones have relatively acidic protons on their a-carbon atoms. Equilibrium constants for enolate formation from sim-... 

Table 6.12. Equilibrium Constants for Enolization of Some Carbonyl Compounds...

Enamines are the nitrogen analogues of enols but their formation from imines is thermodynamically more favourable than enol formation from ketones (Table 1). The equilibrium constant for enol formation is ca. 10 compared with a value of 10 for enamine formation. However, at pH 7 half of the imine exists as the iminium ion and the proportion of enamine present is 10 -fold greater than the proportion of enolate anion. In general, this implies that loss of an electrophile from... 

Equilibrium constants for enolization, 706, 727 for hydration of aldehydes and ketones, 663 table... 

Equilibrium constants for enolization of some carbonyl compounds... 

FIGURE 19.23 The equilibrium constant for enolization of cyclohexadienone is estimated to be greater than 10. ... 

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