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Question #44507

Difficulty: 8/10 | 🟢 Free

The Question

An organic chemist attempts to synthesize a nitrile derivative from an alkyl bromide $\textbf{X}$ ( $\ce{C8H13Br}$ ) using $\ce{NaCN}$ in anhydrous DMSO at 298 K. The reaction fails completely, recovering 100% of the starting material. Attempted solvolysis of $\textbf{X}$ in boiling aqueous ethanol also fails to yield any substitution products.

Spectroscopic analysis confirms that $\textbf{X}$ is a \textbf{primary} alkyl bromide containing a bridged bicyclic framework.

Identify $\textbf{X}$ and the fundamental stereoelectronic reason for its 'mechanistic stalemate'.

Options

A.
\ce{1-(Bromomethyl)bicyclo[2.2.1]heptane}; $S_N2$ is blocked by the cage (neopentyl effect), and $S_N1$ rearrangement yields a Bredt's rule-violating bridgehead carbocation.
B.
\ce{1-Bromobicyclo[2.2.2]octane}; $S_N2$ is blocked by the rigid cage, and $S_N1$ directly forms a highly strained, non-planar bridgehead carbocation.
C.
\ce{2-(Bromomethyl)bicyclo[2.2.1]heptane}; $S_N2$ is blocked by endo-protons, and $S_N1$ yields an unstable anti-aromatic non-classical carbocation.
D.
\ce{7-(Bromomethyl)bicyclo[2.2.1]heptane}; $S_N2$ is blocked by adjacent bridgeheads, and $S_N1$ yields a 7-norbornyl cation that cannot achieve $sp^2$ planarity.

Check Your Answer

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

Cycloalkane Ring Strain|Steric Hindrance in SN2|Reaction Intermediates Stability

Why This Question Is Tricky

Common Mistakes

This question tests the 'mechanistic stalemate' exception where a primary alkyl halide is completely inert to both SN1 and SN2.

Students often memorize 'primary = fast SN2' and 'bridgehead = no SN1/SN2'.

The primary trap is Option B, a classic inert bridgehead halide, which students will rush to select if they ignore the explicit 'primary alkyl bromide' constraint.

The correct answer requires recognizing that a primary halide attached to a bridgehead suffers from extreme neopentyl-like steric hindrance (blocking SN2) and cannot undergo the necessary SN1 rearrangement because shifting an alkyl group would generate a Bredt's rule-violating bridgehead carbocation.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Hydrocarbons - Class 11 Part 2
Concept	Cycloalkane Ring Strain
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free