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 :
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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.
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: \[2,4\text{-Dinitrophenol} < 4\text{-Nitrophenol} < 3\text{-Chlorophenol} < \text{Phenol} < 2,4,5\text{-Trimethylphenol}.\]
