Chapter 18: Enols and Enolates

Compared to simple hydrocarbons, the α-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 :

pKa = 19

pKa » 50

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

Can you think of any other simple examples similar to this situation ?

A very similar case is the comparison of the acidity of carboxylic acids (pK_a = 5) and alcohols (pK_a = 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 carboxylic acid -OH proton is also an α-proton.
 

pKa = 5

pKa = 16

The same pattern occurs with the α-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 stabilisation of the negative charge, the more stable the conjugate base is and therefore the more acidic the parent system.

Typical pK_a values for a ketone and an aldehyde are shown:

pKa = 19	pKa = 17

We can rationalise 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 destabilise anions (you should recall that they stabilise 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 stabilisation of the anion since the charge can be delocalised to 2 electronegative oxygen atoms. As a result, we have an even more acidic pK_a. 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 pK_a = 16. Explain ANSWER

© Dr. Ian Hunt, Department of Chemistry