Acid-Base Properties

The basicity of a carbonyl group can be studied from the acidity of its protonated form. The higher the acidity of the protonated form, the lower the starting carbonyl's basicity.

Basicity of the carbonyl group

The higher the participation of the third resonance form, the more stable (less acidic) the protonated form of a carbonyl group. Hence, the higher the L group's ability to release electrons, the larger the carbonyl group's basicity.

Protonation of an acid derivative R–CO–L at the carbonyl oxygen and resonance forms of the cation, with the positive charge on the oxygen, the carbon and the L group

Resonance form of the protonated derivative with the positive charge on the L group

Can you rationalize in the previous terms the following pK_a values?

Acetyl chloride protonated at the carbonyl oxygen — Acetyl chloride

pK_a = -9.0

Chlorine is highly electronegative thus making the protonated form the least stable in the series. Acyl halides bear the least basic carbonyl group in the series.

Ethyl acetate protonated at the carbonyl oxygen — Methyl acetate

pK_a = -6.5

The alkoxy group can stabilize the positive charge by mesomeric effect and the third resonance form makes full sense.

Acetic acid protonated at the carbonyl oxygen — Acetic Acid

pK_a = -6.0

The fact that, in a protonated carboxylic acid, the first and third resonance forms are equivalent gives extra stabilization. That accounts for the paradox that a carboxylic acid is the most basic species of this series.

Acidity of α-hydrogens

The α hydrogens to a carbonyl group are always feebly but usefully acidic, due to the delocalization of the negative charge. Does the L group have anything to do with this?

Deprotonation of the α-hydrogen of a derivative R–CH₂–CO–L by a base and resonance forms of the enolate

pK_a = 30

Amides have the least acidic α hydrogens due to the strong electron-releasing effect of the NMe₂ group that destabilizes the anionic form.

pK_a = 25

The higher electronegativity of the OMe group allows us to explain the higher acidity of the ester's α hydrogen as compared to amide's.

pK_a = 16

Among the three compounds, the acid chloride displays the lowest pK_a values and thus bears the most acidic α hydrogens as a consequence of the highest electronegativity of Cl. The more electronegative the L group, the higher the stabilization of the anion and the more acidic the α hydrogens.