Enolate anions definition

B.i. Definition of Umpolung. Under normal conditions, a carbonyl group is polarized as shown in 333. The carbonyl carbon is electrophilic and the a-carbon nucleophilic, as in an enolate anion (277). It would be very attractive from a synthetic viewpoint if the polarity of a carbonyl could be reversed, as in 334. 

As mentioned on p. 99, gas-phase reactions are under charge control and, therefore, almost by definition, FO theory is inappropriate for their study. Such a conclusion would be precipitous. Note to begin with that only the anion behaves in an unusual manner, the comportment of its partner being normal. An FO study of gas-phase SN2 reactions (X- + RY —> XR + Y ) is therefore perfectly possible. We can also study the competition of electrophilic sites. On the other hand, FO theory will give questionable conclusions for the regioselectivity of anions (e.g. O-alkylation versus C-alkylation of enolates). For these problems, a more thorough study, requiring in particular transition states determination, is necessary. 

Proton transfer can occur in any medium, depending on the charge type. A solution that contains equal concentrations of a base and its conjugate acid has a pH that is equal to the pAabH (Section 3.2.1). The pH of the medium will be close to the p abH of the anion needed for reaction, because the anion must have a high enough concentration to react at a reasonable rate. As an illustration, if a ketone enolate, pATabH = 19.2, is needed for reaction, the medium will definitely be very basic. 

In the system carbon dioxide-carbonic acid, only the latter would be regarded as an acid in the sense of the Bronsted definition, since carbon dioxide has no proton to lose. There are, however, cases in which two isomeric acids are in equilibrium in solution, each being in equilibrium with the same anion. The commonest example involves keto-enol equilibria. If we write the two isomers as SH (keto) and HS (enol), the equilibrium scheme becomes... 

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