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: Compare through the para group's electronic effect.
The para position transmits electronic effects strongly to the \(-COOH\). A group that withdraws electrons will make the O-H bond release its proton more easily.

Step 2: Effect of each group.
\(-NO_2\) withdraws electrons (both inductive and resonance) \(\Rightarrow\) most acidic. \(-OCH_3\) donates electrons by resonance \(\Rightarrow\) least acidic. Plain benzoic acid lies in between.

Step 3: Pick the strongest.
4-nitrobenzoic acid is the most acidic, option (iii).
\[\boxed{\text{Option (III)}}\]