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Chapter 21: Ester Enolates
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Acidity of a-Hydrogens
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In the following table, the acidity of the H
for various enolate systems and other closely related systems are given.
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You should be able to justify the trends in this data !
Why are the protons adjacent to carbonyl groups acidic ?
As we have advocated before, look
at the stabilization of the conjugate base.
Notice the proximity of the adjacent p system,
and hence the possibility for RESONANCE stabilization by delocalisation
of the negative charge to the more electronegative oxygen atom.
| The more effective the resonance stabilization of the negative charge,
the more stable the conjugate base is and therefore the more acidic the
parent system. |
Let's compare pKa of the common systems: aldehyde pKa = 17, ketone pKa
= 19 and an ester pKa = 25, and try to justify the trend.
The difference between the 3 systems is in the nature of the group attached
to the common carbonyl. The aldehyde has a hydrogen, the ketone an
alkyl- group and the ester an alkoxy- group.
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H atoms are regarded as having no electronic effect : they don't withdraw
or donate electrons.
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Alkyl groups are weakly electron donating, they tend to destabilize
anions (you should recall that they stabilize
carbocations). This is because they will be "pushing" electrons towards
a negative system which is unfavourable electrostatically. Hence,
the anion of a ketone, where there are extra alkyl groups is
less
stable than that of an aldehyde, and so, a ketone is less acidic.
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In the ester, there is also a resonance donation from the alkoxy group
towards the carbonyl that competes with the stabilization of the enolate
charge. This makes the ester enolate less stable than those of aldehydes
and ketones so esters are even less acidic.
