Acids and Bases - Organic Chemistry Concepts: An EFL Approach - page 98

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We can see the similarity between carboxylic acids and carbonyl compounds which
have a hydrogen on the α -carbon. This is the same positional arrangement as the OH
hydrogen in carboxylic acids. The resulting enolate anion is resonance stabilized, and
this explains the relative acidity of propanone with pK a 20. However, the non-equivalent
resonance forms are not as efficient as the equivalent carboxylate resonance forms.
This is because one resonance form for the enolate has the negative charge on a
carbon. Both the symmetrical carboxylate resonance forms have the negative charge
on oxygen. Since oxygen is more electronegative than carbon, it can better carry the
negative charge. So, ethanoic acid has the greater acid strength with pK a 4.7.
For propanone, we cannot draw a resonance form similar to the one for ethanoic
acid. Explain this.
G As a carbon analog of a carboxylic acid, propanone has no lone pair for delocalization
into the C ] O. The only way to get a pair of electrons is to lose a proton. This causes the
transfer of the proton to the oxygen as shown below.
These two structures are not resonance forms because the atomic arrangement
has changed. Instead, they are the equilibrium between two structural isomers. This
equilibrium is an example of tautomerism between the keto and enol forms of propanone.
Do you think that esters and amides show similar α -hydrogen acidity to the ketone
in the reaction?