Question

Explain why the ${\mathbf{pK}}_{\mathbf{a}}$of compound A is lower than the ${\mathbf{pK}}_{\mathbf{a}}$’s of both compounds B and C.

Short answer

In compound A the oxygen atom is more electronegative than the nitrogen atom in compound C. As a result, the oxygen atom will result in stronger inductive effect in compound A.

The oxygen atom is at a distance from the carboxylic acid group. Due to which the inductive effect is less in compound B and it is a weaker acid as compared to compound A.

Step-by-step solution

Strength of an acid

The strength of an acid can be determined by the ${\mathrm{pK}}_{\mathrm{a}}$value of the acid. The stronger acid always has the lower ${\mathrm{pK}}_{\mathrm{a}}$value and the weaker acid has the higher ${\mathrm{pK}}_{\mathrm{a}}$ value.The deprotonation of acid can take place by using a base whose conjugate acid has a higher ${\mathbf{pK}}_{\mathbf{a}}$ value.

Lower pKa value of compound A as compared to compound B and C.

In compound A the oxygen atom is more electronegative than the nitrogen atom in compound C. As a result, the oxygen atom will result in the stronger inductive effect in compound A.

Compound A

Compound C

As result, the conjugate base of compound A will be much more stabilized. It will make compound A more acidic than compound C.

On the other hand, compound A is more acidic than compound B. This can be attributed to the inductive effect. In compound A, the oxygen atom is closer to the carboxylic acid group.

Compound A

Compound B

The electronegative oxygen atom can cause a stronger electron-withdrawing inductive effect in compound A. On the other hand, the oxygen atom is at a distance from the carboxylic acid group. Due to which the inductive effect is less in compound B and it is a weaker acid as compared to compound A.