Reaction of stabilized carbanions enolates with alkyl halides enolate alkylation

1 Reaction of stabilized carbanions (enolates) with alkyl halides (enolate alkylation) 

According to the concept of the hard-soft acid base, the carbon is considered more nucleophilic than the oxygen, thus more likely to react than the oxygen atom because the leaving 

The values of heat of formation (AH ) of the reactants, products and transition states for O-alkylation and C-alkylation are reported in Table 3.1. 

The activation energies, calculated from the change in enthalpy in going from the reactant to the transition state, are -1-20.3 and -1-15.6 kcal mol for O-alkylation and C-alkylation, respectively. The overall enthalpy change of the reactions, obtained from the differences in the heat of formation between the reactant and the product, is -1-5.7 and —13.0 kcal mol for O-alkylation and C-alkylation, respectively. These results predict that the product obtained for C-alkylation is the preferred product because (i) the activation barrier is smaller (lower in energy) than that in O-alkylation and (ii) the reaction is exothermic while O-alkylation is an endothermic process. 

When ethoxide is used as a base to abstract a-proton, it could react with the alkyl halide (Sn2) to form ether. However, the acid-base equilibrium with the diketone prefers the more stable, less basic enolate over the ethoxide, essentially consuming the ethoxide. Thus, the very stable enolates from 1,3-dicarbonyl compounds are able to undergo Sn2 reactions with alkyl halides without competition from the alkoxide catalyst. 

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