Assertion (A) : p-nitrophenol is more acidic than phenol.
Reason (R) : Nitro group is an electron-withdrawing group, it stabilises phenoxide ion by dispersal of negative charge.
Reason (R) : Nitro group is an electron-withdrawing group, it stabilises phenoxide ion by dispersal of negative charge.
Show Hint
The effect of EWG is most pronounced at ortho and para positions due to resonance.
Always look for charge dispersal to determine the stability of the conjugate base.
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Assertion (A) is false, but Reason (R) is true.
The Correct Option is A
Solution and Explanation
• Acidity of phenols depends on the stability of the phenoxide ion formed after losing a proton.
• Electron-withdrawing groups (EWG) increase acidity by stabilizing the negative charge on the oxygen through inductive and resonance effects.
Step 1: Evaluate the Assertion
p-Nitrophenol has a nitro group \( (-\text{NO}_2) \) at the para position.
The nitro group is a strong electron-withdrawing group.
It facilitates the release of the proton and stabilizes the resulting phenoxide ion better than phenol itself.
Thus, p-nitrophenol is more acidic. Assertion is True.
Step 2: Evaluate the Reason
The nitro group exerts both \( -\text{I} \) and \( -\text{M} \) effects.
At the para position, the \( -\text{M} \) effect effectively delocalizes the negative charge of the phenoxide ion.
This dispersal of charge leads to higher stability. Reason is True.
Step 3: Check for logical connection
The reason specifically explains why the nitro group makes the molecule more acidic (via stabilization of the ion).
Therefore, the reason is the correct explanation.
The final answer is (A).
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