Question

The increasing order of pKa for the following phenols is 
(A) 2,4-Dinitrophenol 
(B) 4-Nitrophenol 
(C) 2,4,5-Trimethylphenol 
(D) Phenol 
(E) 3-Chlorophenol 
Choose the correct answer from the option given below :

• A. (C), (E), (D), (B), (A)
• B. (C), (D), (E), (B), (A)
• C. (A), (E), (B), (D), (C)
• D. (A), (B), (E), (D), (C)

Hint:

For analyzing pKa trends:
• Electron-withdrawing groups increase acidity (lower pKa).
• Electron-donating groups decrease acidity (higher pKa).
• Consider the position and number of substituents for their effect on the phenoxide ion.

Answer 1

The Correct Option is D

1. Understanding \(pK_a\):
   Lower \(pK_a\) values indicate stronger acids.
   Electron-withdrawing groups (e.g., \(-\text{NO}_2\), \(-\text{Cl}\)) stabilize the phenoxide ion, increasing acidity (lower \(pK_a\)).
   Electron-donating groups (e.g., \(-\text{CH}_3\)) destabilize the phenoxide ion, decreasing acidity (higher \(pK_a\)).
2. Analysis of Substituents:
   2,4-Dinitrophenol: Two \(-\text{NO}_2\) groups strongly withdraw electrons, making it the most acidic (lowest \(pK_a\)).
   4-Nitrophenol: One \(-\text{NO}_2\) group withdraws electrons, making it less acidic than 2,4-Dinitrophenol.
   3-Chlorophenol: \(-\text{Cl}\) is a moderate electron-withdrawing group, making it less acidic than \(-\text{NO}_2\) compounds but more acidic than phenol.
   Phenol: The parent compound without additional substituents is less acidic than substituted derivatives with electron withdrawing groups.
   2,4,5-Trimethylphenol: Three \(-\text{CH}_3\) groups are electron-donating, making it the least acidic (highest \(pK_a\)).
Final Order:
\[2,4\text{-Dinitrophenol} < 4\text{-Nitrophenol} < 3\text{-Chlorophenol} < \text{Phenol} < 2,4,5\text{-Trimethylphenol}.\]