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

To determine the correct order of acidic strength among the given compounds, we need to understand how substituents on the aromatic ring affect the acidity of phenols. 

Concept: The acidity of phenols is affected by the substituents present on the aromatic ring. Electron-withdrawing groups (EWGs) increase acidity by stabilizing the negative charge on the phenoxide ion, while electron-donating groups (EDGs) decrease acidity by destabilizing it.

Comparison:

• Phenol (I): The -OH group is directly bonded to the benzene ring. No additional substituents, hence moderate acidity.
• p-Cresol (II): It has a methyl group (an EDG) at the para position, which decreases the acidity compared to phenol.
• m-Nitrophenol (III): The meta position of the nitro group (an EWG) stabilizes the phenoxide ion, making it more acidic than phenol. However, the meta effect is weaker than the para effect.
• p-Nitrophenol (IV): The nitro group at the para position has a strong electron-withdrawing effect, greatly increasing the acidity compared to phenol.

Conclusion: Based on the above explanation, the order of acidic strength is \( \text{IV} > \text{III} > \text{I} > \text{II} \), where p-Nitrophenol (IV) is the most acidic due to the strong para effect of the nitro group, followed by m-Nitrophenol (III), phenol (I), and finally p-Cresol (II) with the least acidity due to the presence of the electron-donating methyl group. Hence, the correct answer is \( \text{IV} > \text{III} > \text{I} > \text{II} \).