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Question #44400

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The Question

An organic chemist aims to synthesize the diaryl ether 1-methoxy-4-(4-nitrophenoxy)benzene (Compound $\textbf{W}$ ) and the benzyl aryl ether 1-fluoro-4-((4-nitrophenoxy)methyl)benzene (Compound $\textbf{Z}$ ).

The following synthetic pathways are evaluated: \textbf{Path I:} \ce{1-chloro-4-methoxybenzene} + sodium 4-nitrophenoxide $\xrightarrow{\text{DMF, } 150^\circ\text{C}}$ \textbf{Path II:} \ce{1-chloro-4-nitrobenzene} + sodium 4-methoxyphenoxide $\xrightarrow{\text{DMF, } 150^\circ\text{C}}$ \textbf{Path III:} \ce{1-(chloromethyl)-4-fluorobenzene} + sodium 4-nitrophenoxide $\xrightarrow{\text{DMF, } 25^\circ\text{C}}$

Which of the following statements correctly identifies the successful pathways and their corresponding mechanisms?

Options

A.
Path II yields $\textbf{W}$ via an addition-elimination ( $S_NAr$ ) mechanism involving a Meisenheimer complex, and Path III yields $\textbf{Z}$ via an $S_N2$ mechanism at the benzylic carbon. Path I fails due to the absence of a strong electron-withdrawing group.
B.
Path I yields $\textbf{W}$ via an $S_NAr$ mechanism, and Path III yields $\textbf{Z}$ via an $S_NAr$ mechanism at the fluorinated carbon. Path II fails due to the deactivating effect of the methoxy group on the nucleophile.
C.
Both Path I and Path II yield $\textbf{W}$ via a benzyne intermediate, while Path III yields $\textbf{Z}$ via an $S_N1$ mechanism due to the stability of the 4-fluorobenzyl cation.
D.
Path II yields $\textbf{W}$ via an $S_N2$ mechanism on the aromatic ring, and Path III yields $\textbf{Z}$ via an $S_NAr$ mechanism. Path I fails due to steric hindrance.

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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

Williamson Ether Synthesis Constraints|Nucleophilic Aromatic Substitution (SNAr)

Why This Question Is Tricky

Common Mistakes

This question tests the mechanistic boundaries between Williamson Ether Synthesis ( $S_N2$ ) and Nucleophilic Aromatic Substitution ( $S_NAr$ ).

A common student trap is assuming that any alkoxide/phenoxide will react with any organohalide to form an ether.

The problem highlights that $S_N2$ is strictly forbidden on $sp^2$ hybridized aromatic carbons.

It requires students to recognize that $S_NAr$ necessitates specific electronic activation (a strong EWG like $-NO_2$ at the ortho or para position) to stabilize the intermediate Meisenheimer complex.

Furthermore, Path III introduces a chemoselectivity trap, requiring the realization that an unactivated aryl halide will not undergo $S_NAr$ , whereas a benzylic halide will rapidly undergo $S_N2$ under mild conditions.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Alcohols, Phenols, and Ethers - Class 12
Concept	Williamson Ether Synthesis Constraints
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free