Question

Which of the following is most acidic?
(I) 4-methoxybenzoic acid (\( -COOH \) with para \( -OCH_3 \))
(II) benzoic acid
(III) 4-nitrobenzoic acid (\( -COOH \) with para \( -NO_2 \))

• A. (I)
• B. (II)
• C. (III)
• D. None of these

Hint:

Electron-withdrawing groups like \( -NO_2 \) at the para position increase acid strength; \( -OCH_3 \) decreases it.

Answer 1

The Correct Option is C

Step 1: Concept of acid strength.
A carboxylic acid is stronger when its conjugate base (carboxylate ion) is more stable. Electron-withdrawing groups (EWG) pull electron density away and stabilise the carboxylate, increasing acidity. Electron-donating groups (EDG) do the opposite.

Step 2: Classify the para substituents.
\( -NO_2 \) is a strong electron-withdrawing group. \( -OCH_3 \) is an electron-donating group (via resonance). \( -H \) (benzoic acid) is the reference.

Step 3: Rank acidity.
EWG \( -NO_2 \) raises acidity, EDG \( -OCH_3 \) lowers it. Hence 4-nitrobenzoic acid > benzoic acid > 4-methoxybenzoic acid.

Conclusion: Most acidic is (III), 4-nitrobenzoic acid. Option (iii).
\[\boxed{\text{4-nitrobenzoic acid}}\]