Return to Contents  Chapter 21: Ester Enolates Ch 21 contents
Acidity of a-Hydrogens
pKa data for various systems   active methylene compound acidity

Why are the protons adjacent to carbonyl groups acidic ?
As we have advocated before, look at the stabilisation of the conjugate base.
Notice the proximity of the adjacent p system, and hence the possibility for RESONANCE stabilisation by delocalisation of the negative charge to the more electronegative oxygen atom.

stabilisation of the conjugate base by resonance

The more effective the resonance stabilisation 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.

comparing aldehydes and ketones
resonance in an ester enolate
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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© Dr. Ian Hunt, Department of Chemistry, University of Calgary