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Question #44169

Difficulty: 8/10 | 🟢 Free

The Question

Consider the electrophilic aromatic substitution of the following compounds via diazo coupling with \ce{2,4-dinitrobenzenediazonium tetrafluoroborate} in an acetate buffer (pH = 5.0) at 273 K:

I. Phenol II. 2-Nitrophenol III. 3-Nitrophenol IV. 4-Nitrophenol

Let $k_I, k_{II}, k_{III},$ and $k_{IV}$ be their respective initial rates of reaction. Identify the correct decreasing order of these rates.

Options

A.
$k_I > k_{III} > k_{IV} > k_{II}$
B.
$k_I > k_{III} > k_{II} > k_{IV}$
C.
$k_I > k_{II} > k_{IV} > k_{III}$
D.
$k_{III} > k_I > k_{IV} > k_{II}$

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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

Hydrogen Bonding Competition|Electrophilic Substitution Activation/Deactivation|Coupling Reactivity

Why This Question Is Tricky

Common Mistakes

The question tests the competition between static electronic effects (+M, -I, -M), steric hindrance, and dynamic intramolecular interactions (Hydrogen bonding) in Electrophilic Aromatic Substitution.

Students often fall into the trap of predicting 2-nitrophenol to be more reactive than 4-nitrophenol because electrophilic attack on 2-nitrophenol occurs at the sterically unhindered para position, whereas attack on 4-nitrophenol must occur at the sterically hindered ortho position.

However, the 'Hidden Variable' is the strong intramolecular hydrogen bond (chelation) in 2-nitrophenol.

This H-bond 'locks' the lone pair of the hydroxyl oxygen, drastically reducing its +M (mesomeric) electron-donating ability to stabilize the Wheland intermediate.

Consequently, the competitive H-bonding overrides the expected steric advantage, making 2-nitrophenol the least reactive isomer.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Chemical Bonding and Molecular Structure - Class 11 Part 1
Concept	Hydrogen Bonding Competition
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free