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Compared to simple hydrocarbons, the a-protons
adjacent to carbonyl groups are much more acidic and can be removed by
common bases (e.g. HO-, RO- etc.).
For example, compare the acidity of propanone and propane :
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Question
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.
A very similar case is the comparison of the acidity of carboxylic acids
(pKa = 5) and alcohols (pKa = 16). The acid is more acidic since
the negative charge can be delocalised to a second electronegative oxygen
atom. This delocalisation makes the carboxylate more stable (more favourable).
This delocalisation is not possible in the alcohol. Infact the -OH
proton is also an a-proton.
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The same pattern occurs with the a-hydrogens in aldehydes and ketones... a resonance structure can be drawn with the negative charge relocated on the 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.
Typical pKa values for a ketone and an aldehyde are shown:
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We can rationalize the trend by comparing the two structures, the difference
simply being the alkyl- group versus the hydrogen.
Since 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 anions of ketone where there are extra alkyl groups are less
stable than those of aldehyde, and so, ketones are less acidic.
In some cases there could be H atoms that are adjacent to 2 carbonyl groups. This means that there is more resonance stabilization of the anion since the charge can be delocalised to 2 electronegative oxygen atoms. As a result, we have an even more acidic pKa. These type of compounds are sometimes called "active methylenes".
Question:
A different system, but related type of scenario, is the hydrocarbon
1,3-cyclopentadiene which has a pKa = 16. Can you explain this ?
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