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Question #45052

Difficulty: 8/10 | 🟢 Free

The Question

Consider the chiral substrate \textbf{X}, which is (2 $S$ , 3 $S$ )-2-bromo-3-phenylbutane. \textbf{X} is subjected to two separate reactions under different solvent conditions:

\textbf{Reaction 1:} \textbf{X} is treated with an excess of anhydrous \ce{KF} in Dimethyl Sulfoxide (DMSO) at 60 $^\circ$ C to give major product \textbf{P}.

\textbf{Reaction 2:} \textbf{X} is treated with an excess of \ce{KI} in Methanol at 60 $^\circ$ C to give major product \textbf{Q}.

Identify the major products \textbf{P} and \textbf{Q}.

Options

A.
\textbf{P} = ( $E$ )-2-phenyl-2-butene ; \textbf{Q} = (2 $R$ , 3 $S$ )-2-iodo-3-phenylbutane
B.
\textbf{P} = ( $Z$ )-2-phenyl-2-butene ; \textbf{Q} = (2 $R$ , 3 $S$ )-2-iodo-3-phenylbutane
C.
\textbf{P} = (2 $R$ , 3 $S$ )-2-fluoro-3-phenylbutane ; \textbf{Q} = ( $E$ )-2-phenyl-2-butene
D.
\textbf{P} = (2 $S$ , 3 $S$ )-2-fluoro-3-phenylbutane ; \textbf{Q} = (2 $S$ , 3 $S$ )-2-iodo-3-phenylbutane

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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

Inductive Effect (I-Effect)|C-X Bond Polarization and Polarizability|SN2 Stereochemical Inversion

Why This Question Is Tricky

Common Mistakes

This question tests the 'Solvent Polarity Effect' on the reactivity of halide ions, specifically the inversion of their nucleophilic and basic properties.

Students often memorize that fluoride is a poor nucleophile/base based on protic solvent trends.

However, in polar aprotic solvents (DMSO), fluoride becomes a 'naked' ion and a powerful base, shifting the mechanism for secondary halides from S_N2 to E2.

Conversely, iodide in protic solvents remains a strong nucleophile and weak base, favoring S_N2.

The problem further traps students with the stereospecificity of the E2 elimination (requiring anti-periplanar geometry to yield the E-alkene from the threo diastereomer) and the inversion of configuration in S_N2.

Quick Facts About This Question

Question Information

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