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 determine the acidity order of the –OH group in the given compounds, we need to understand the effect of substituents on the phenol ring.

1. Phenol Acidity: Phenols are acidic because they can donate a hydrogen ion (proton) from the hydroxyl group, resulting in a phenoxide ion. The stability of the phenoxide ion determines the acidity of phenols. 
2. Effect of Substituents:
   • Electron-withdrawing groups (EWG) like \(-\text{NO}_2\) increase acidity by stabilizing the negative charge on the phenoxide ion through resonance or induction.
   • Electron-donating groups (EDG) like \(-\text{OCH}_3\) decrease acidity by destabilizing the phenoxide ion.
3. Analysis of Each Compound:
   • (A) Bu – OH: A simple butanol with no resonance stabilizing effects, therefore, least acidic.
   • (B) \(p-\text{NO}_2\text{-C}_6\text{H}_4\text{-OH}\): Contains a strong electron-withdrawing \(-\text{NO}_2\) group, significantly increasing acidity.
   • (C) \(p-\text{OCH}_3\text{-C}_6\text{H}_4\text{-OH}\): Contains an electron-donating \(-\text{OCH}_3\) group, decreasing acidity.
   • (D) Phenol: Standard reference for acidic strength in phenols.
   • (E) \(2,4-\text{NO}_2\text{-C}_6\text{H}_3\text{-OH}\): The presence of two \(-\text{NO}_2\) groups greatly enhances the acidity due to strong electron-withdrawing effects.
4. Conclusion: Based on the effects explained above, the correct order of acidity is:

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

Answer 2

The acidity of phenols increases with the presence of electron-withdrawing groups (like -NO₂) and decreases with electron-donating groups (like -CH₃).

• (A): Contains a butyl group (electron-donating), making it the least acidic.
• (C): Contains a methoxy group (CH₃O-) (electron-donating), slightly more acidic than (A).
• (D): Simple phenol (C₆H₅OH), with no additional electron-withdrawing or donating groups, has moderate acidity.
• (B): Contains an -OH group with a chlorine substituent (electron-withdrawing) on the aromatic ring, making it more acidic than (D).
• (E): Contains an -OH group with a nitro substituent (NO₂) which is strongly electron-withdrawing, making it the most acidic.

Thus, the correct order of acidity is $(A) < (C) < (D) < (B) < (E)$.