A 1,3-dicarbonyl is often used to form enolate carbanions.
Reasoning: A 1,3-dicarbonyl is particularly acidic because there are two carbonyls to delocalize negative charge. Therefore, makes it easier to form the enolate carbanion.
Note: We have seen that protonation of an enolate at oxygen leads to an enol. The enol, an unstable isomer of an aldehyde or ketone, rapidly converts into the carbonyl system: It tautomerizes. These isomers are called enol and keto tautomers.
Overview ↓: Enol–keto tautomerism proceeds by either acid or base catalysis.
Figure ↑: In the acid-catalyzed process, the enol form is protonated at the double-bonded carbon
away from the hydroxy-bearing neighbor. Moreover, the resulting cation is
resonance-stabilized by the attached hydroxy group, and inspection of the corresponding
resonance form reveals it to be simply the protonated carbonyl compound. Deprotonation
then gives the product.
Figure ↑: Base simply removes the proton from the enol oxygen, reversing the initial protonation. Subsequent (and slower) C-protonation furnishes the thermodynamically more stable keto form.
General Note ↑: Both the acid- and base-catalyzed enol–keto interconversions occur rapidly in solution
whenever there are traces of the required catalysts. Remember that although the keto form
(usually) predominates, the enol-to-keto conversion is reversible and the mechanisms by
which the keto form equilibrates with its enol counterpart are the exact reverse of the
preceding two schemes.
Note ↑: The pKa values of aldehyde and ketone -hydrogens range from 16 to 21, much lower than
the pKa values of ethene (44) or ethyne (25), but comparable with those of alcohols (15–18).
Strong bases can therefore remove an -hydrogen. The anions that result are known as
enolate ions or simply enolates.
Why are aldehydes and ketones relatively acidic?
We know that acid strength is enhanced
by stabilization of the conjugate base. In the enolate ion, the inductive effect
of the positively polarized carbonyl carbon strongly stabilizes the negative charge at the
-position. Aldehydes are stronger acids than ketones because their carbonyl carbon bears a
larger partial positive charge. Further strong stabilization is provided by delocalization of charge onto the electronegative oxygen, as described by the resonance forms
just pictured. The effect of delocalization is also reflected in the electrostatic potential map
of the acetone enolate shown down below (on an attenuated scale), which exhibits negative
charge (red) on the -carbon as well as on the oxygen. An example of quantitative enolate
formation is the deprotonation of cyclohexanone (pKa = 19) by lithium diisopropylamide (LDA,
pKa of amine = 36).
