Enols and enolate ions

The reaction rate does not vary significantly with the acidity indicating that the enols and enolate ions have about the same reactivity d Average velocity constants for the reactions of Br2 and Br, e Toullec and Dubois, 1973... 

Mechanism 22-1 Alpha Substitution 1045 Mechanism 22-2 Addition of an Enolate to Ketones and Aldehydes (a Condensation) 1046 Mechanism 22-3 Substitution of an Enolate on an Ester (a Condensation) 1046 22-2 Enols and Enolate Ions 1046... 

Alpha substitutions and condensations of carbonyl compounds are some of the most common methods for forming carbon-carbon bonds. These types of reactions are common in biochemical pathways, particularly in the biosynthesis and metabolism of carbohydrates and fats. A wide variety of compounds can participate as nucleophiles or electrophiles (or both) in these reactions, and many useful products can be made. We begin our study of these reactions by considering the structure and formation of enols and enolate ions. 

Supporting evidence for the mechanism comes from the observation that the bromination and iodination proceed at the same rates. The deuterium exchange is also comparable in absolute rate. Very extensive work with the optically active sec-butyl phenyl ketone, C2H5—CH(CH3)COC6H6, has shown that the acid-catalyzed iodination, bromination, and inversion have identical rates. The base-catalyzed, OD, rates of deuteration and inversion have also been shown to be equal. If the enol and enolate ion can be considered to be planar about the a carbon atom, then these results provide very strong support for the slow enolization step. In fact it is difficult to find any other reasonable interpretation of the data. The enol mechanism is also compatible with the well-known susceptibility of H atoms, in the alpha position to one or more C==0 groups, to substitution reactions. 

Other reactions involving enols and enolate ions. 176... 

Figure 2. The pH dependence of the relative content (%) of ketone, enol, and enolate-ion for the wild-type (1) and mutant (2) firefly luciferases...

The enols and enolate ions of ketones and similar substances will react not only with halogens but with many other substances, and in particular the many addition and condensation reactions undergone by carbonyl compounds in presence of basic catalysts can be referred to an... 

Show how enols and enolate ions act as nucleophiles. Give mechanisms for acid-catalyzed and base-catalyzed keto-enol tautomerisms. 

Write the structures of every enol and enolate ion that can arise from each of the carbonyl compounds illustrated in Problem 32. 

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