Question:
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.
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.
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Electron-withdrawing groups such as
\[
-NO_2,\; -CN,\; -CHO,\; -COOH
\]
increase the acidity of phenols by stabilising the phenoxide ion.
Electron-donating groups such as
\[
-CH_3,\; -OCH_3,\; -NH_2
\]
decrease acidity by destabilising the conjugate base.
Updated On: Jun 29, 2026
- 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.
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The Correct Option is A
Solution and Explanation
Concept:
The acidity of phenolic compounds depends primarily upon the stability of the conjugate base formed after the removal of a proton.
When phenol loses a proton \((H^+)\), it forms the phenoxide ion.
\[
C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+
\]
Greater stability of the phenoxide ion results in greater acidity of the corresponding phenol.
Electron-withdrawing groups increase acidity by stabilising the negative charge present on the conjugate base, whereas electron-donating groups decrease acidity by increasing electron density and destabilising the conjugate base.
Step 1: Examining the Assertion. The Assertion states that \[ p\text{-nitrophenol} \] is more acidic than phenol. The structure of p-nitrophenol contains a nitro group \((-NO_2)\) attached at the para position of the benzene ring. The nitro group is a strongly electron-withdrawing substituent. Because of this electron-withdrawing nature, it stabilises the conjugate base formed after deprotonation. As a result, p-nitrophenol releases a proton more readily than phenol. Therefore, the Assertion is true.
Step 2: Understanding the effect of the nitro group on acidity. The nitro group exhibits both \[ -I \] (inductive electron-withdrawing effect) and \[ -R \] (resonance electron-withdrawing effect). Due to these effects, electron density is pulled away from the benzene ring and from the oxygen atom carrying the negative charge. This leads to greater delocalisation of charge and increased stability of the phenoxide ion. \[ p\text{-Nitrophenoxide Ion} \] is therefore more stable than ordinary phenoxide ion. Since a more stable conjugate base corresponds to a stronger acid, p-nitrophenol is more acidic than phenol.
Step 3: Examining the Reason. The Reason states that the nitro group is an electron-withdrawing group and stabilises the phenoxide ion by dispersal of negative charge. This statement is scientifically correct. The negative charge generated after deprotonation is delocalised more effectively due to the presence of the nitro group. Hence, the Reason is true.
Step 4: Determining whether the Reason explains the Assertion. The Assertion says that p-nitrophenol is more acidic than phenol. The Reason explains that the nitro group stabilises the phenoxide ion by withdrawing electron density and dispersing the negative charge. Since the stability of the conjugate base is the direct reason for increased acidity, the Reason correctly explains the Assertion.
Final Conclusion: Assertion (A) is true. Reason (R) is true. Reason (R) correctly explains Assertion (A). \[ \boxed{\text{Option (A)}} \]
Step 1: Examining the Assertion. The Assertion states that \[ p\text{-nitrophenol} \] is more acidic than phenol. The structure of p-nitrophenol contains a nitro group \((-NO_2)\) attached at the para position of the benzene ring. The nitro group is a strongly electron-withdrawing substituent. Because of this electron-withdrawing nature, it stabilises the conjugate base formed after deprotonation. As a result, p-nitrophenol releases a proton more readily than phenol. Therefore, the Assertion is true.
Step 2: Understanding the effect of the nitro group on acidity. The nitro group exhibits both \[ -I \] (inductive electron-withdrawing effect) and \[ -R \] (resonance electron-withdrawing effect). Due to these effects, electron density is pulled away from the benzene ring and from the oxygen atom carrying the negative charge. This leads to greater delocalisation of charge and increased stability of the phenoxide ion. \[ p\text{-Nitrophenoxide Ion} \] is therefore more stable than ordinary phenoxide ion. Since a more stable conjugate base corresponds to a stronger acid, p-nitrophenol is more acidic than phenol.
Step 3: Examining the Reason. The Reason states that the nitro group is an electron-withdrawing group and stabilises the phenoxide ion by dispersal of negative charge. This statement is scientifically correct. The negative charge generated after deprotonation is delocalised more effectively due to the presence of the nitro group. Hence, the Reason is true.
Step 4: Determining whether the Reason explains the Assertion. The Assertion says that p-nitrophenol is more acidic than phenol. The Reason explains that the nitro group stabilises the phenoxide ion by withdrawing electron density and dispersing the negative charge. Since the stability of the conjugate base is the direct reason for increased acidity, the Reason correctly explains the Assertion.
Final Conclusion: Assertion (A) is true. Reason (R) is true. Reason (R) correctly explains Assertion (A). \[ \boxed{\text{Option (A)}} \]
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