Acetone, enolization hydration

Rate and equilibrium constants have been determined for the aldol condensation of a, a ,a -trifluoroacetophenone (34) and acetone, and the subsequent dehydration of the ketol (35) to the cis- and fraw -isomeric enones (36a) and (36b)." Hydration of the acetophenone, and the hydrate acting as an acid, were allowed for. Both steps of the aldol reaction had previously been subjected to Marcus analyses," and a prediction that the rate constant for the aldol addition step would be 10" times faster than that for acetophenone itself is borne out. The isomeric enones are found to equilibrate in base more rapidly than they hydrate back to the ketol, consistent with interconversion via the enolate of the ketol (37), which loses hydroxide faster than it can protonate at carbon. 

Comparisons of structurally related hydroxy- and methoxy-substituted cations show that hydroxy is more stabilizing by between 4 and 5 log units. This difference was recognized 20 years ago by Toullec who compared pifas for protonation of the enol of acetophenone and its methyl ether145 (-4.6 and 1.3, respectively) based on a cycle similar to that of Scheme 15, but with the enol replacing the hydrate, and a further cycle relating the enol ether to a corresponding dimethyl acetal and methoxycarbocation.146 Toullec concluded, understandably but incorrectly, that there was an error in the pA a of the ketone (over which there had been controversy at the time).147,148 In a related study, Amyes and Jencks noted a difference of 105-fold in reactivity in the nucleophilic reaction with water of protonated and O-methylated acetone and concluded that the protonated acetone lacked a full covalent bond to... 

Keto-enol tautomerism was discussed in Chapter 10 (Section 10.6) in connection with the hydration reaction of alkynes, including oxymercuration or hydroboration. The carbonyl form is more stable, and for most aldehydes and ketones only a tiny amount of enol is present. Acetone (2), for example, exists primarily as the ketone, and experiments show only 1.5 x 10 % enol. This experiment titrated acetone with diatomic bromine and measured the extent of reaction, which converted 2 to l-bromo-2-propanone (a-bromoacetone, 4). The second product in this reaction is HBr. By this experiment, the enol content of acetone—and presumably of other simple ketones—is remarkably small. 

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