Thermodynamic stability keto-enol forms

The aci-form of the nitro group is frequently claimed in pure chemistry. However, only ad-nitromethane appears to have been comprehensively studied as an isolated species15. Ionized keto-enol systems are characterized by reversal of the relative stabilities of the single species compared to their neutral counterparts. Thus, the ionized enols are generally the thermodynamically more favoured tautomers by approximately 15-20 kcalmol-1, the... 

We have established that enols are, in general, less stable than the keto form of the molecule. We might hope to see stable enols if we changed that situation by adding some feature to the molecule that stabilized the enol thermodynamically. Or we might try to create an enol that would revert only slowly to the keto form—in other words, it would be kinetically stable. We shall look at this type first. 

Introduction of a nitro group into a hydrocarbon considerably enhances the thermodynamic acidity of the C—H bond. For example, nitroethane has pK 8.60 [99]. The effect of the nitro group is comparable to the effect of the two keto groups in acetylacetone (pK 8.9) [17]. The nitrocarbanion (or nitronate ion) is strongly stabilized by inductive and mesomeric electron withdrawal. In solution the ionization (73) is slightly complicated by the presence of the aci-nitro isomer, similar to the enolic form of a ketone, viz. 

Recalling the discussion of the dynamics of proton transfer in Section 9.3.7, protonation and deprotonation of heteroatoms is faster than with carbon. Therefore, the mechanism given in Scheme 10.9 is, as with many classic electron-pushing schemes, a simplification. Protonation actually occurs first on the oxygen to make the enol form (Ecp 10.35), but the higher thermodynamic stability of the keto form relative to the enol form ultimately leads to the carbonyl product (see Section 11.1). 

Both Z and E isomeric enol lactones undergo photoisomerization to yield mixtures of isomers (5,14,87) in which the thermodynamically more stable one prevails. It is the Z form in hydrastine series (5) and the E isomer in the more hindered narcotine series (87). Relative stabilities of isomeric enol lactones (98 versus 99 and 101 versus 102) were determined by comparing their rates of methanolysis. Keto esters of type 126 were formed (87). It turned out that both ( )-N-methylhydrastine enol lactone (99) and (Z)-narceine enol lactone (101) solvolyzed faster than their geometric partners. 

Because we are left with a carbanion after the Claisen condensation, acidification of the reaction mixture is required in order to form the neutral P-keto ester. What prevents this product, thermodynamically unstable with respect to the starting ester molecules, from re-forming the starting esters Certainly the Claisen condensation is reversible. But the p-keto ester that is formed in the basic conditions is much more likely to form the resonance-stabilized enolate than go back to starting material. And once the stable enolate is formed it is not likely to react with alkojdde, a nucleophile + nucleophile reaction. And after acidification there is no base present Without the base, there can be no reverse Claisen condensation, and the product ester is obtained (Fig. 19.104). 

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