Question

Rearrange the following in the increasing order of acidic strength. (i) benzoic acid (ii) p-methoxybenzoic acid (iii) o-methyoxybenzoic acid

• A. i < ii < iii
• B. iii < i < ii
• C. ii < i < iii
• D. iii < ii < i

Answer 1

Answer: (C)

ii < i < iii

Answer 2

The acidic strength of benzoic acids is influenced by the electron-donating or withdrawing nature of substituents.

• p-methoxybenzoic acid (ii): The methoxy group (-OCH₃) at the para position is electron-donating via resonance (+R effect), destabilizing the carboxylate anion and reducing acidity.
• Benzoic acid (i): This is the reference compound.
• o-methoxybenzoic acid (iii): The methoxy group at the ortho position has a strong inductive electron-withdrawing effect (-I) and exhibits the ortho effect, which can stabilize the carboxylate anion (e.g., via intramolecular hydrogen bonding), increasing acidity.

Thus, the order of increasing acidic strength is p-methoxybenzoic acid < Benzoic acid < o-methoxybenzoic acid, or (ii) < (i) < (iii).