Keto-enol tautomerization reactions catalysis

The addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

Using these relations, the rate coefficients for specific and general acid catalysis, Ah and Aha, of any keto-enol tautomeric reaction can be predicted from the appropriate free energy of reaction ArG°. The required... 

Keto-enol tautomerism of carbon) ] compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that Joses H+ from its a carbon to yield a neutral enol (Figure 22.1). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H+ to form an alkene (Section 11.10). 

We must now consider the kinetic analysis of catalysed reactions involving two proton transfers. These include the so-called prototropic isomerizations, notably keto-enol tautomerism. The simplest mechanisms for catalysis by acids and bases are as follows ... 

The importance of catalysis of the keto-enol tautomerization is brought home by calculations of the energy of the hypothetical transition states for direct intramolecular proton shift between carbon and oxygen. For acetone, it is 53 kcal mol This would require temperatures of 500°C for the reaction to occur without a catalyst. 

Since keto-enol tautomerism is accelerated by both acids and bases (the case of general acid-base catalysis), the reaction rate increases with both pH increase and decrease (V-shaped dependence). For example, the rate of acetone enolization in an acetate buffer is related to concentrations of die reactants by the following expression (H20,298 K) ... 

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