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Question #44384

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The Question

Consider the acid dissociation of $o$ -nitrophenol (I) and $p$ -nitrophenol (II) at $298\text{ K}$ . Let $K_a(\text{I})$ and $K_a(\text{II})$ represent their respective acid dissociation constants. Which of the following correctly identifies the relative acidity order in water (polar protic solvent) versus benzene (non-polar aprotic solvent)?

Options

A.
Water: $K_a(\text{II}) > K_a(\text{I})$ ; Benzene: $K_a(\text{I}) > K_a(\text{II})$
B.
Water: $K_a(\text{I}) > K_a(\text{II})$ ; Benzene: $K_a(\text{II}) > K_a(\text{I})$
C.
Water: $K_a(\text{II}) > K_a(\text{I})$ ; Benzene: $K_a(\text{II}) > K_a(\text{I})$
D.
Water: $K_a(\text{I}) > K_a(\text{II})$ ; Benzene: $K_a(\text{I}) > K_a(\text{II})$

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Phenol Acidity vs. Substituents|Basicity-Solvation Competition|Inductive Effect Trends

Why This Question Is Tricky

Common Mistakes

Students often memorize that p-nitrophenol is more acidic than o-nitrophenol due to the intramolecular hydrogen bond 'trapping' the proton in the ortho isomer.

However, they fail to recognize that acidity is a thermodynamic competition heavily dependent on solvent stabilization of the conjugate base.

In water, the p-nitrophenoxide anion is extensively solvated, making (II) more acidic.

In non-polar solvents, external solvation is absent.

Here, the intrinsic stability of the anion governs acidity.

The o-nitrophenoxide anion is significantly stabilized by the strong electrostatic attraction between the phenoxide oxygen and the adjacent positively charged nitrogen of the nitro group, along with a stronger inductive (-I) effect.

This intrinsic stabilization outweighs the energy cost of breaking the intramolecular H-bond, flipping the acidity order so that (I) > (II) in aprotic media.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Alcohols, Phenols, and Ethers - Class 12
Concept	Phenol Acidity vs. Substituents
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free