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Question #44619

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following two independent reactions performed at $298\text{ K}$ :

Reaction I: $(2R, 4S)\text{-2-bromo-4-methylhexane}$ is treated with $1\text{ equivalent}$ of $\ce{NaCN}$ in $\ce{DMSO}$ to yield the major product $\mathbf{P}$ .

Reaction II: $(4S)\text{-4-methylhexan-2-one}$ is treated with excess $\ce{HCN}$ in the presence of a catalytic amount of $\ce{KCN}$ to yield product $\mathbf{Q}$ .

Which of the following correctly describes the stereochemical nature of products $\mathbf{P}$ and $\mathbf{Q}$ ?

Options

A.
$\mathbf{P}$ : exclusively $(2S, 4S)$ ; $\mathbf{Q}$ : racemic mixture
B.
$\mathbf{P}$ : exclusively $(2S, 4S)$ ; $\mathbf{Q}$ : unequal mixture of diastereomers
C.
$\mathbf{P}$ : unequal mixture of diastereomers; $\mathbf{Q}$ : racemic mixture
D.
$\mathbf{P}$ : exclusively $(2R, 4S)$ ; $\mathbf{Q}$ : unequal mixture of diastereomers

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

SN2 Stereochemical Inversion|Cyanohydrin Stereochemistry|Optical Activity and Enantiomers

Why This Question Is Tricky

Common Mistakes

This problem tests the distinction between substrate-controlled stereospecificity ( $\text{S}_\text{N}2$ ) and addition to a prochiral center in a chiral molecule.

Students often fall into the trap of associating planar carbonyl additions strictly with 'racemic mixtures'.

However, when a pre-existing chiral center is present, the faces of the carbonyl are diastereotopic, not enantiotopic.

Consequently, nucleophilic attack yields an unequal mixture of diastereomers.

Furthermore, it verifies the student's ability to track absolute configuration $(R/S)$ during an $\text{S}_\text{N}2$ inversion where the priority of the incoming nucleophile matches the leaving group.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	SN2 Stereochemical Inversion
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free