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Question #44644

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The Question

Consider the following reaction sequence: Step 1: 1,4-Dichlorobenzene is treated with a nitrating mixture (\ce{HNO3}/\ce{H2SO4}) to yield the major mononitrated product \textbf{A}. Step 2: \textbf{A} is reacted with 1 equivalent of anhydrous \ce{KF} in DMSO at 120 $^{\circ}$ C to form compound \textbf{B}. Step 3: \textbf{B} is treated with an equimolar mixture of $o$ -anisidine and $p$ -anisidine (1 equivalent total) in the presence of \ce{K2CO3} in ethanol at 298 K to yield the major product \textbf{C}.

Which of the following statements correctly identifies \textbf{C} and the underlying mechanistic rationale?

Options

A.
\textbf{C} is 4-chloro- $N$ -(4-methoxyphenyl)-2-nitroaniline. Step 2 proceeds via a nitro-stabilized Meisenheimer complex at the ortho-position. In Step 3, $p$ -anisidine outcompetes $o$ -anisidine due to the absence of the ortho-effect, and F is displaced faster than Cl.
B.
\textbf{C} is 4-chloro- $N$ -(2-methoxyphenyl)-2-nitroaniline. Step 2 proceeds via a nitro-stabilized Meisenheimer complex. In Step 3, $o$ -anisidine outcompetes $p$ -anisidine due to the stronger inductive (+I) effect of the closer methoxy group, displacing F.
C.
\textbf{C} is 4-fluoro- $N$ -(4-methoxyphenyl)-2-nitroaniline. Step 2 replaces the meta-chlorine due to lower steric hindrance. In Step 3, $p$ -anisidine displaces Cl because chloride is a better leaving group than fluoride in nucleophilic substitutions.
D.
\textbf{C} is 4-chloro- $N$ -(4-methoxyphenyl)-2-nitroaniline. Step 2 proceeds via an aryne intermediate due to steric inhibition by the ortho-nitro group. In Step 3, $p$ -anisidine attacks the less hindered aryne carbon.

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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

Aryl Halide Resonance|Nucleophilic Aromatic Substitution (SNAr)|Ortho-Effect in Anilines

Why This Question Is Tricky

Common Mistakes

The question tests three critical exceptions and traps in SNAr.

First, identifying that in 1,4-dichloro-2-nitrobenzene, the C4 chlorine is meta to the nitro group and thus unactivated, contrary to visual intuition.

Second, the kinetic trap where students assume Cl is a better leaving group than F, failing to recognize that the addition step is rate-determining in SNAr.

Third, the 'Ortho-Effect' trap: students misapply simple +I inductive trends to conclude o-anisidine is a stronger nucleophile, ignoring the spatial and steric factors that make ortho-substituted anilines exceptionally weak bases and nucleophiles.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Aryl Halide Resonance
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free