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Question #44041

Difficulty: 8/10 | 🟢 Free

The Question

A primary alkyl halide $X$ ( $\ce{C5H11Br}$ ) is treated with potassium phthalimide in $N,N$ -dimethylformamide (DMF) at $353\text{ K}$ . The reaction fails to yield the expected $N$ -alkylphthalimide. However, when $X$ is heated with aqueous $\ce{AgNO3}$ , it forms a pale yellow precipitate and yields an alcohol $Y$ . Alcohol $Y$ produces immediate turbidity upon treatment with Lucas reagent ( $\ce{ZnCl2}$ / conc. $\ce{HCl}$ ). Identify $X$ and the primary kinetic barrier preventing its reaction with potassium phthalimide.

Options

A.
1-Bromo-2,2-dimethylpropane; Severe steric hindrance at the $\beta$ -carbon prevents the required $S_N2$ backside attack.
B.
1-Bromo-2-methylbutane; The phthalimide anion acts as a strong base, leading to an $E2$ elimination pathway.
C.
1-Bromo-2,2-dimethylpropane; The highly stable neopentyl carbocation intermediate favors an $S_N1$ pathway, bypassing the Gabriel synthesis.
D.
2-Bromo-2-methylbutane; Steric hindrance at the $\alpha$ -carbon blocks the $S_N2$ trajectory of the phthalimide nucleophile.

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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

Gabriel Phthalimide Synthesis|Nucleophilic Acyl Substitution

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Neopentyl Exception' in $S_N2$ reactions.

Students often memorize the heuristic that 'primary alkyl halides undergo Gabriel phthalimide synthesis' and fail to account for steric hindrance at the $\beta$ -carbon.

By selecting neopentyl bromide, the question forces the student to recognize that the $S_N2$ transition state requires a backside attack that is kinetically prohibited by the massive steric bulk of the adjacent \textit{tert}-butyl group.

The secondary trap (Option C) tests the misconception that neopentyl systems form stable primary carbocations, whereas they actually undergo rapid rearrangement, and Option D traps students who ignore the 'primary alkyl halide' constraint.

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