Question

Phthalic acid and isophthalic acid have protons on two carboxy groups that can be removed with base.

(a) Explain why the ${\mathbf{pK}}_{\mathbf{a}}$for loss of the first proton (${\mathbf{pK}}_{\mathbf{a}\mathbf{1}}$) is lower for phthalic acid than isophthalic acid.

(b) Explain why the ${\mathbf{pK}}_{\mathbf{a}}$for loss of the second proton ( ${\mathbf{pK}}_{\mathbf{a}\mathbf{2}}$) is higher for phthalic acid than isophthalic acid.

Short answer

a. The loss of the first proton of phthalic acid is quicker than isophthalic acid. Therefore, phthalic acid has a lower ${\mathrm{pK}}_{\mathrm{a}}$ ( ${\mathrm{pK}}_{\mathrm{a}1}$= 2.9) than isophthalic acid ( ${\mathrm{pK}}_{\mathrm{a}1}$=3.7).

b. The ${\mathrm{pK}}_{\mathrm{a}}$ value for the loss of the second proton ofphthalic acid is higher than isophthalic acid due to intramolecular hydrogen bonding.

Step-by-step solution

pKa value

The ${\mathbf{pK}}_{\mathbf{a}}$value is defined as the “negative base-10 log of the acid dissociation constant ${\mathbf{K}}_{\mathbf{a}}$” ofa solution.

Smaller the ${\mathbf{pK}}_{\mathbf{a}}$ value, the stronger is the acid.

Comparing the pKa value of phthalic acid and isophthalic acid 

a.

Structure of phthalic acid and isophthalic acid

• The COOH group accepts the electron density by its resonance effect, and this destabilizes the conjugate base, making the acid less acidic than isophthalic acid.

• Thus, the loss of the first proton of phthalic acid is quicker than isophthalic acid.

• Therefore, phthalic acid has a lower ${\mathrm{pK}}_{\mathrm{a}}$ (${\mathrm{pK}}_{\mathrm{a}1}$ = 2.9) than isophthalic acid (${\mathrm{pK}}_{\mathrm{a}1}$=3.7).

b.

Intramolecular interaction

• Intramolecular hydrogen bonding stabilizes the conjugate base, making the phthalic acid more acidic than isophthalic acid.

• Thus, the ${\mathrm{pK}}_{\mathrm{a}}$ value for the loss of the second proton ofphthalic acid is higher than isophthalic acid.