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Question #45043

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The Question

Consider the reaction of 2-methoxytetrahydro-4H-pyran-4-one with excess \ce{NaCN} and dilute \ce{HCl} at 298 K to form a cyanohydrin. The reaction is allowed to reach equilibrium. Which of the following statements correctly describes the major diastereomer formed and the governing stereochemical principles?

Options

A.
The major diastereomer has the \ce{-OH} group \textit{trans} to the \ce{-OMe} group. The \ce{-OMe} group is axial due to the anomeric effect, and thermodynamic control dictates that the bulkier \ce{-OH} group occupies the equatorial position.
B.
The major diastereomer has the \ce{-OH} group \textit{cis} to the \ce{-OMe} group. The axial \ce{-OMe} group sterically hinders axial attack, forcing the incoming \ce{CN^-} to attack equatorially under kinetic control.
C.
The major diastereomer has the \ce{-OH} group \textit{cis} to the \ce{-OMe} group. The \ce{-OMe} group occupies an equatorial position to minimize steric hindrance, directing the \ce{CN^-} to the axial position.
D.
A racemic mixture of both diastereomers is formed in equal proportions because the $sp^2$ hybridized carbonyl carbon allows equal probability of nucleophilic attack from either face.

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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

Hybridization and Geometry|Cyanohydrin Stereochemistry|Aldol Condensation Directionality

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of the Anomeric Effect, Nucleophilic Addition Trajectories, and Kinetic vs.

Thermodynamic Control.

The 'Hidden Variable' is the reversibility of cyanohydrin formation.

Students often memorize that nucleophiles attack cyclohexanones axially (Felkin-Anh/torsional strain) or equatorially (if sterically hindered).

Here, the axial -OMe sterically blocks axial attack, leading to a kinetic preference for equatorial attack (yielding an axial -OH).

The 'Chemical Exception' is that because the reaction is reversible, it escapes kinetic control.

The thermodynamic product dominates, driven by the larger A-value of the -OH group compared to the -CN group, forcing -OH into the equatorial position.

This results in a trans relationship between the -OH and -OMe groups, trapping students who only apply kinetic steric arguments (Option B).

Quick Facts About This Question

Question Information

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