Depto. Química Orgánica

ACID-BASE PROPERTIES OF ANHYDRIDES

Image Link UAM Excelencia UAM Facultad Ciencias UAM
15/07/2017
page counter
Where I am Problems and exercises Molecular panel Home
Basicity of the Carbonyl Group
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.
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.
Image
Image
Can you rationalize in the previous terms the following pKa values?
Image Acetyl chloride

pKa = -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.

Image Acetic
Anhydride

pKa = -8.5
The electronegativities of Cl and O-C=O should be very much alike. But in an anhydride there are two C=O groups and the protonation probability doubles. On the other hand, the protonated species should be additionally stabilized by the hydrogen bonding shown. All that should account for the slightly higher basicity of anhydrides as compared to acyl chlorides.

Image Ester

pKa = -6.5
The alkoxy group can stabilize the positive charge by mesomeric effect and the third resonance form makes full sense.

Image Acid

pKa = -6.0
The fact that, in a protonated carboxylic acid, the first and third resonance forms are equivalent gives an extra of stabilization. That avails for the paradox that a carboxylic acid is the most basic species of this series.

Alpha-Hydrogen's Acidity
The alpha hydrogens to a carbonyl group are always feebly but usefully acidic, due to the delocalization of the negative charge. Has anything to do the L group with this?
Image
Image DMF

pKa = 30
Amides have the least acidic alpha hydrogens due to the strong electron-releasing effect of the NMe2 group that destabilizes the anionic form.

Image Methyl acetate

pKa = 25
The higher electronegativity of the OMe group allows us to explain the higher acidity of the ester's alpha hydrogen as compared to amide's.

Image Acetyl chloride

pKa = 16
Among the these compounds, the acid chloride displays the lowest pKa values and thus bears the most acidic alpha 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 alpha hydrogens.

Image Acetic Anhydride

pKa = 16
Anhydrides and acyl chlorides are very much alike because the electronegativities of Cl and O-C=O groups are very similar.