14.6 : Acidity and Basicity of Carboxylic Acid Derivatives

Unlike carboxylic acids, the derivatives are not true acids because they lack an acidic proton attached to a functional group. However, the presence of α hydrogens makes them weakly acidic.

The relative acidic strength depends on the extent of resonance stabilization of the conjugate base.

Nitriles are weakly acidic, as the negative charge is on a less electronegative nitrogen atom, destabilizing the conjugate base.

In amides, the negative charge is on a more electronegative oxygen, making the conjugate base relatively more stable and amides slightly more acidic.

Since stabilization increases with the electronegativity of the substituent, acid chlorides are the most acidic.

Similarly, the relative basicity can be assessed by the stability of the conjugate acid.

Nitriles are the least basic. Here, the resonance structure is destabilized due to an incomplete octet at the cyano carbon.

Acid chlorides are slightly more basic; however, chlorine's higher electronegativity lowers the stability of the protonated form.

The stability of the conjugate acid increases with a decrease in the electronegativity of the substituent, making amides the most basic.