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Question #44484

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The Question

Consider the electrophilic mononitration of 4-chloro-1,1'-biphenyl ( $\textbf{X}$ ) using a standard nitrating mixture ( $\ce{HNO3}$ / $\ce{H2SO4}$ ) at 298 K to form the major product $\textbf{Y}$ . Let $E_{a,A}$ and $E_{a,B}$ represent the activation energies for the formation of the arenium ion intermediate via para-attack on the chlorinated ring (Ring A) and the unsubstituted ring (Ring B), respectively. Which of the following statements correctly identifies $\textbf{Y}$ and accurately describes the kinetic and electronic factors governing its formation?

Options

A.
$\textbf{Y}$ is 4-chloro-4'-nitro-1,1'-biphenyl. $E_{a,A} > E_{a,B}$ because the strong $-I$ effect of chlorine raises the activation energy for attack on Ring A relative to Ring B, even though chlorine's $+M$ effect provides resonance stabilization to the Ring A para-sigma complex.
B.
$\textbf{Y}$ is 4-chloro-2-nitro-1,1'-biphenyl. $E_{a,A} < E_{a,B}$ because the $+M$ effect of chlorine strongly activates the ortho position of Ring A, overcoming the $-I$ effect and making it kinetically more reactive than Ring B.
C.
$\textbf{Y}$ is 4-chloro-2'-nitro-1,1'-biphenyl. $E_{a,A} > E_{a,B}$ because the 4-chlorophenyl group exerts a strong $+M$ effect on Ring B, selectively stabilizing the ortho-sigma complex over the para-sigma complex due to proximity.
D.
$\textbf{Y}$ is 4-chloro-3-nitro-1,1'-biphenyl. $E_{a,A} < E_{a,B}$ because the $-I$ effect of chlorine strongly deactivates the ortho/para positions of Ring A, making the meta position of Ring A the most kinetically accessible site overall.

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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

Aromaticity (Huckel’s Rule)|Electrophilic Substitution Activation/Deactivation|Aryl Halide Resonance

Why This Question Is Tricky

Common Mistakes

This question tests the 'Inductive vs.

Resonance' conflict in aryl halides within a competitive intramolecular system.

Students often memorize that halogens are ortho/para directing and mistakenly conflate this orientation effect (driven by $+M$ resonance stabilization of the intermediate) with overall reactivity.

The hidden trap is realizing that the strong $-I$ effect of chlorine deactivates its own ring (Ring A) so severely that the electrophile preferentially attacks the adjacent, less deactivated ring (Ring B).

This requires distinguishing between the relative stability of sigma complexes (which dictates regiochemistry) and the absolute activation energy of the rate-determining step (which dictates chemoselectivity).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Hydrocarbons - Class 11 Part 2
Concept	Aromaticity (Huckel’s Rule)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free