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.
14.6:
Acidity and Basicity of Carboxylic Acid Derivatives
Carboxylic acids are the strongest among organic acids, as they readily lose the hydroxyl proton to form a resonance-stabilized carboxylate ion. In comparison, the acid derivatives lack acidic hydrogens directly attached to a functional group. In these compounds, the acidic nature arises from their ability to lose α hydrogens, making them weakly acidic.
The relative acidic strength of the derivatives can be explained based on the extent of resonance stabilization of the conjugate base. The higher the stability of the conjugate base, the stronger the acid. The stability of the conjugate base increases from nitriles to acid halides, as shown below:
As a result, nitriles are the weakest acids in the series, whereas acid halides are the strongest.
Similarly, the basicity can be evaluated based on the stability of the conjugate acid, which increases from nitriles to amides as shown below:
Accordingly, nitriles are the weakest bases and amides the strongest.