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Question #45046

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The Question

Consider the following substituted phenols in aqueous medium at $298\text{ K}$ : (I) 4-nitrophenol (II) 3,5-dimethyl-4-nitrophenol (III) Phenol (IV) 2,6-di-\textit{tert}-butylphenol

Due to phenomena such as Steric Inhibition of Resonance (SIR) and steric hindrance to solvation, the standard 2D electronic effects ( $+I, -M$ ) are often overridden by 3D conformational changes. Based on these stereoelectronic constraints, what is the correct decreasing order of their acid dissociation constants ( $K_a$ )?

Options

A.
$\text{(I)} > \text{(II)} > \text{(III)} > \text{(IV)}$
B.
$\text{(I)} > \text{(III)} > \text{(II)} > \text{(IV)}$
C.
$\text{(II)} > \text{(I)} > \text{(IV)} > \text{(III)}$
D.
$\text{(I)} > \text{(II)} > \text{(IV)} > \text{(III)}$

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Hybridization and Geometry|Inductive Effect (I-Effect)|Phenol Acidity vs. Substituents

Why This Question Is Tricky

Common Mistakes

This question tests the 3D conformational analysis of substituted phenols, specifically Steric Inhibition of Resonance (SIR) and steric hindrance to solvation.

Students often apply 2D electronic effects blindly.

The trap is twofold: (1) In 3,5-dimethyl-4-nitrophenol, the $\ce{-NO2}$ group is forced out of plane, losing its $-M$ effect but retaining its $-I$ effect (so it remains more acidic than phenol). (2) In 2,6-di-\textit{tert}-butylphenol, the massive ortho groups force the $\ce{-OH}$ / $\ce{-O^-}$ out of planarity, destroying the resonance stabilization of the phenoxide ion itself and blocking solvation, making it exceptionally weakly acidic.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Hybridization and Geometry
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free