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Question #44002

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The Question

Consider the modified Kiliani-Fischer synthesis of D-erythrose. D-Erythrose is treated with aqueous \ce{NaCN} at pH 9.5 to form a cyanohydrin mixture (\textbf{P}). \textbf{P} is catalytically hydrogenated (\ce{H2}, \ce{Pd-BaSO4}) to yield an $\alpha$ -hydroxy imine intermediate (\textbf{Q}). Prior to the standard acidic hydrolysis, \textbf{Q} is inadvertently exposed to a mildly basic buffer (pH 8.5) for an extended period, leading to a loss of the initial kinetic diastereomeric ratio. It is subsequently hydrolyzed to the aldopentose mixture (\textbf{R}). Which of the following correctly identifies the key intermediate responsible for the epimerization at C-2 during the basic delay, and the final composition of \textbf{R}?

Options

A.
Enamine tautomer ( $1$ -aminoalk- $1$ -en- $2$ -ol); \textbf{R} is a mixture of $(2R,3R,4R)$ - and $(2S,3R,4R)-2,3,4,5$ -tetrahydroxypentanal.
B.
$\alpha$ -Carbanion stabilized by the imine nitrogen; \textbf{R} is a mixture of $(2R,3S,4R)$ - and $(2S,3S,4R)-2,3,4,5$ -tetrahydroxypentanal.
C.
$2$ -Aminoalk- $1$ -en- $1$ -ol intermediate; \textbf{R} is a mixture of $(3R,4R)$ - and $(3S,4R)-1,3,4,5$ -tetrahydroxypentan- $2$ -one.
D.
Reversible cyanohydrin dissociation to an aldehydic enolate; \textbf{R} is a mixture of $(2R,3R,4R)$ - and $(2S,3R,4R)-2,3,4,5$ -tetrahydroxypentanal.

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Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Nucleophilic Acyl Substitution|Kiliani-Fischer Chain Extension|Epimerization vs Anomerization

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of intermediate stability and epimerization in the Kiliani-Fischer synthesis.

Students typically memorize that the reaction yields two epimers due to non-stereoselective cyanide addition.

However, this problem introduces a basic delay at the imine stage, triggering an imine-enamine tautomerization (a nitrogen-analog of the Lobry de Bruyn-van Ekenstein transformation).

It traps students who fail to recognize the acidity of the $\alpha$ -proton in the $\alpha$ -hydroxy imine and forces them to rigorously map the IUPAC stereochemical configurations of the resulting aldopentoses from the D-erythrose precursor.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Nucleophilic Acyl Substitution
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free