Transcript Acidity of Aldehydes and Ketones: Enolate Ions

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Acidity of Aldehydes and Ketones: Enolate Ions
The pKa values of the -hydrogens of aldehydes and ketones range from 16 to 21,
comparable to those of alcohols (15-18).
Strong bases can remove  hydrogens leading to anions called enolate ions or
enolates.
The enolate resonance hybrid possesses partial negative charges on both carbon
and oxygen and may attack electrophiles at either position.
A species that can attack at two different sites to give two different products is
called ambident.
Alkylation of cyclohexanone enolate with 3-chloropropene occurs at the carbon
atom, while protonation occurs at the oxygen atom.
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Keto-Enol Equilibria
An enol equilibrates with its keto form in acidic or basic solution.
Keto-enol tautomerism is the interconversion of the thermodynamically more
stable keto form and the enol form of a carbonyl compound.
Keto-enol tautomerism proceeds by either base or acid catalysis.
Either equilibration is fast and reversible in solution in the presence of the
required catalyst.
Substituents can shift the keto-enol equilibrium.
For ordinary aldehydes and ketones, only traces of the enol form are present. The
enol form is less stable by 8-12 kcal mol-1.
However, for acetaldehyde, the enol form is about 100-times more stable than
that of acetone because the less substituted aldehyde carbonyl is more stable
than the more substituted ketone carbonyl.
Enol formation leads to deuterium exchange and stereoisomerization.
Treatment of a ketone with traces of acid or base in D2O solvent leads to the
exchange of all the  carbons:
The number of  hydrogens in a molecule can be readily determined by following
the disappearance of the 1H NMR signal as the hydrogens are sequentially
replaced by deuterium.
Enolization also leads to the easy interconversion of stereoisomers at -carbons.
The trans isomers are sterically more stable.
When an -hydrogen is part of a stereocenter, keto-enol tautomerization can lead
to racemization in the presence of a basic or acidic catalyst.
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Halogenation of Aldehydes and Ketones
Aldehydes and ketones react with halogens at their -carbons.
The extent of halogenation depends upon whether acidic or basic catalysis is
used.
The rate of halogenation is independent of the halogen concentration. The ratedetermining step involves the carbonyl substrate:
Further halogenation is retarded because the electron-withdrawing halogen
substituent makes enolization more difficult than in the original substrate.
Singly halogenated product molecules are not attacked until all of the starting
aldehyde or ketone has been used up.
Base catalyzed halogenation proceeds by formation of an enolate ion, which then
attacks the halogen.
The electron-withdrawing power of the halogen substituent makes the remaining
-hydrogens more acidic and complete halogenation of the -carbon occurs,
leaving unreacted starting material (when insufficient halogen is employed).