Question

Correct order of acidic strength: (I) Phenol, (II) p-Cresol, (III) m-Nitrophenol, (IV) p-Nitrophenol

• A. \( \text{IV}>\text{III}>\text{I}>\text{II} \)
• B. \( \text{III}>\text{IV}>\text{I}>\text{II} \)
• C. \( \text{IV}>\text{I}>\text{III}>\text{II} \)
• D. \( \text{III}>\text{I}>\text{IV}>\text{II} \)

Hint:

Acidity of phenols increases with electron-withdrawing substituents and decreases with electron-donating substituents. Resonance effect operates at ortho and para positions, not at meta.

Answer 1

The Correct Option is A

Step 1: Electron-withdrawing groups increase acidity by stabilizing the phenoxide ion, while electron-donating groups decrease acidity.
Step 2: The nitro group (\(-\mathrm{NO_2}\)) is a strong electron-withdrawing group. At the para position, it stabilizes the phenoxide ion by both \(-I\) and \(-R\) effects, making p-nitrophenol the most acidic.
Step 3: In m-nitrophenol, the nitro group shows only the \(-I\) effect (no resonance interaction), so it is less acidic than p-nitrophenol but more acidic than phenol.
Step 4: p-Cresol contains a methyl group (\(-\mathrm{CH_3}\)), which is electron-donating and destabilizes the phenoxide ion, making it the least acidic. \[ \text{Acidic strength: } \text{IV}>\text{III}>\text{I}>\text{II} \]