Question

The ${\mathbf{pK}}_{\mathbf{a}}$of acetamide $\left({\mathrm{CH}}_3{\mathrm{CONH}}_2\right)$is 16. Draw the structure for its conjugate base and explain why acetamide is less acidic than ${\mathbf{CH}}_{\mathbf{3}}\mathbf{COOH}$.

Short answer

The resonating structures of the acetate ion are more stabilized than the acetamide ion. Therefore, acetic acid is a stronger acid than acetamide.

Step-by-step solution

Factors affecting the acidity

The acidity of the compounds depends upon the inductive effect and the resonance effect of the conjugate base of that compound.

• The compounds become more acidic due to the electron-withdrawing inductive effect of the groups attached to the conjugate base. The electron-withdrawing groups such as halogens increase the acidity of the compounds.

• The stability of the conjugate base of an acid also helps to determine the acidic strength of a compound. The more stable the conjugate base, the more is the acidity of that compound.

Comparison of the acidic strength of acetamide and acetic acid

The acetamide is less acidic as compared to the acetic acid.

The structure of the conjugate base of acetamide is as shown below:

Structure of conjugate base of acetamide

In the case of acetamide, the negative charge is present on the oxygen in one resonating structure and on the nitrogen atom in the other resonating structure.

Since the oxygen atom is more electronegative than the nitrogen atom, the presence of the negative charge on the oxygen atom is much more stabilized as compared to the presence of the negative charge on the nitrogen atom.

Structure of the conjugate base of acetate ion

Therefore, the resonating structures of the acetate ion are more stabilized as compared to the acetamide ion. Therefore, acetic acid is a stronger acid than acetamide.