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Question #44042

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The Question

An optically active primary amine $\textbf{Q}$ ( $\ce{C8H11N}$ ) with an $(R)$ -configuration is synthesized in high yield from an amide $\textbf{P}$ using $\ce{Br2 / KOH}$ at $343\text{ K}$ . An attempt to synthesize $\textbf{Q}$ via the Gabriel phthalimide synthesis using the appropriate chiral alkyl bromide $\textbf{R}$ and potassium phthalimide in DMF at $373\text{ K}$ fails, yielding a hydrocarbon $\textbf{S}$ ( $\ce{C8H8}$ ) as the major product. Identify $\textbf{P}$ , $\textbf{R}$ , and $\textbf{S}$ .

Options

A.
$\textbf{P}$ = $(R)$ -2-phenylpropanamide; $\textbf{R}$ = $(S)$ -(1-bromoethyl)benzene; $\textbf{S}$ = styrene
B.
$\textbf{P}$ = $(S)$ -2-phenylpropanamide; $\textbf{R}$ = $(R)$ -(1-bromoethyl)benzene; $\textbf{S}$ = styrene
C.
$\textbf{P}$ = $(R)$ -2-phenylpropanamide; $\textbf{R}$ = $(R)$ -(1-bromoethyl)benzene; $\textbf{S}$ = styrene
D.
$\textbf{P}$ = $(S)$ -2-phenylpropanamide; $\textbf{R}$ = $(S)$ -(1-bromoethyl)benzene; $\textbf{S}$ = ethylbenzene

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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

Gabriel Phthalimide Synthesis|Hoffmann Bromamide Rearrangement

Why This Question Is Tricky

Common Mistakes

This question tests the 'Lethal Collision' trap between nucleophilic substitution and molecular rearrangement.

Students often mechanically apply the Gabriel synthesis to any primary amine without considering the steric limitations of the alkyl halide precursor (secondary halides undergo E2 elimination with the bulky phthalimide base).

Furthermore, it rigorously tests the stereochemical outcomes of both reactions: retention of configuration in the Hoffmann rearrangement versus inversion of configuration in the SN2 pathway, requiring precise Cahn-Ingold-Prelog (CIP) priority tracking.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Gabriel Phthalimide Synthesis
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free