Question:
Choose the correct order of acidity of the following phenols:
(I) m-nitrophenol (II) p-cresol (III) p-nitrophenol (IV) phenol
Choose the correct order of acidity of the following phenols:
(I) m-nitrophenol (II) p-cresol (III) p-nitrophenol (IV) phenol
(I) m-nitrophenol (II) p-cresol (III) p-nitrophenol (IV) phenol
Updated On: Apr 7, 2025
- (III) > (I) > (IV) > (II)
- (II) > (IV) > (III) > (I)
- (I) > (II) > (III) > (IV)
- (IV) > (II) > (III) > (I)
- (III) > (II) > (I) > (IV)
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The Correct Option is A
Approach Solution - 1
The acidity of phenols is influenced by substituents that either stabilize or destabilize the phenoxide ion.
- p-Nitrophenol (III) is the most acidic due to the electron-withdrawing nitro group at the para position, which stabilizes the phenoxide ion through resonance.
- m-Nitrophenol (I) is less acidic than p-nitrophenol, but more acidic than phenol due to the electron-withdrawing nitro group, albeit in the meta position, which has less resonance effect.
- Phenol (IV) is more acidic than p-cresol (II) because the hydroxyl group in phenol is a stronger electron-withdrawing group compared to the methyl group in p-cresol.
- p-Cresol (II) is the least acidic because the methyl group is an electron-donating group, which destabilizes the phenoxide ion.\\
The correct option is (A) : (III) > (I) > (IV) > (II)
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Approach Solution -2
The acidity of phenols is affected by the substituents attached to the benzene ring. Electron-withdrawing groups increase acidity, while electron-donating groups decrease acidity.
Let's analyze the given phenols:
- (I) m-nitrophenol: Nitro group (\(-NO_2\)) is electron-withdrawing and increases acidity. When it is at the meta position, the resonance effect is negligible, but it still exerts an inductive (-I) effect.
- (II) p-cresol: Methyl group (\(-CH_3\)) is electron-donating and decreases acidity.
- (III) p-nitrophenol: Nitro group (\(-NO_2\)) is electron-withdrawing and increases acidity. The effect is stronger than in m-nitrophenol because resonance stabilization of the conjugate base is maximized when the nitro group is in the para position.
- (IV) Phenol: No substituents, so it serves as a reference point.
Based on these considerations, we can arrange the phenols in order of increasing acidity:
p-cresol < phenol < m-nitrophenol < p-nitrophenol
Therefore, the correct order of acidity is:
(III) > (I) > (IV) > (II)
Therefore, the answer is (III) > (I) > (IV) > (II).
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