Question

Choose the correct order of acidity of the following phenols:
(I) m-nitrophenol (II) p-cresol (III) p-nitrophenol (IV) phenol

• A. (III) > (I) > (IV) > (II)
• B. (II) > (IV) > (III) > (I)
• C. (I) > (II) > (III) > (IV)
• D. (IV) > (II) > (III) > (I)
• E. (III) > (II) > (I) > (IV)

Answer 1

The Correct Option is A

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)

Answer 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).