JEE Chemistry 2023 Question with Solution

Given: We need the increasing order of $pK_a$ for the listed phenols.

Find: The correct option matching the acidity trend.

Lower $pK_a$ means stronger acidity. Electron-withdrawing groups stabilize the phenoxide ion and therefore lower $pK_a$, while electron-donating groups destabilize the phenoxide ion and raise $pK_a$.

From the given explanation:

Thus, the increasing order is exactly this sequence.

Among the provided options, there is a mismatch because the source options list individual compounds instead of full ordered sequences, while the solution states The Correct Option is D. Therefore, based on the solution, the answer is taken as D, though the options as provided are inconsistent with the question format.

Therefore, the correct option is D.

Given: Substituent effects on phenol acidity.

Find: Why the order of $pK_a$ follows the stated trend.

• 2,4-Dinitrophenol has two $-\text{NO}_2$ groups, which strongly withdraw electrons and stabilize the phenoxide ion the most, so it has the lowest $pK_a$.
• 4-Nitrophenol has one $-\text{NO}_2$ group, so it is less acidic than 2,4-dinitrophenol but more acidic than phenol.
• 3-Chlorophenol has a $-\text{Cl}$ group, which shows electron-withdrawing inductive effect, so it is more acidic than phenol.
• Phenol is the reference compound.
• 2,4,5-Trimethylphenol has three $-\text{CH}_3$ groups, which donate electron density and make the phenoxide ion less stable, so it has the highest $pK_a$.

Hence the order is: