Question

The ascending order of acidity of –OH group in the following compounds is: 

Choose the correct answer from the options given below :

• A. \( (A) < (D) < (C) < (B) < (E)\)
• B. \( (C) < (A) < (D) < (B) < (E)\)
• C. \( (C) < (D) < (B) < (A) < (E)\)
• D. \( (A) < (C) < (D) < (B) < (E)\)

Answer 1

The Correct Option is D

To establish the acidity hierarchy of the –OH group in the provided substances, an understanding of how substituents impact the phenol ring is necessary.

1. Phenol Acidity: Phenols exhibit acidity by releasing a hydrogen ion (proton) from their hydroxyl group, yielding a phenoxide ion. The stability of this phenoxide ion dictates the phenol's acidity.
2. Substituent Effects:
   • Electron-withdrawing groups (EWG), such as \(-\text{NO}_2\), augment acidity by stabilizing the phenoxide ion's negative charge via resonance or inductive effects.
   • Electron-donating groups (EDG), like \(-\text{OCH}_3\), diminish acidity by destabilizing the phenoxide ion.
3. Individual Compound Analysis:
   • (A) Bu – OH: A basic butanol lacking resonance stabilization, rendering it the least acidic.
   • (B) \(p-\text{NO}_2\text{-C}_6\text{H}_4\text{-OH}\): Possesses a potent electron-withdrawing \(-\text{NO}_2\) group, leading to a substantial increase in acidity.
   • (C) \(p-\text{OCH}_3\text{-C}_6\text{H}_4\text{-OH}\): Contains an electron-donating \(-\text{OCH}_3\) group, which reduces acidity.
   • (D) Phenol: Serves as the baseline for comparing phenolic acid strengths.
   • (E) \(2,4-\text{NO}_2\text{-C}_6\text{H}_3\text{-OH}\): The presence of two \(-\text{NO}_2\) groups significantly amplifies acidity due to pronounced electron-withdrawing characteristics.
4. Conclusion: Based on the aforementioned effects, the sequence of increasing acidity is:

\((A) < (C) < (D) < (B) < (E)\)