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Question #44991

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The Question

D-Lyxose is subjected to the Kiliani-Fischer chain extension. The intermediate epimeric cyanohydrins are hydrolyzed and dehydrated to yield a mixture of two $\gamma$ -lactones. To successfully isolate the corresponding aldohexoses, the lactones must be selectively reduced using Reagent $X$ . However, a student mistakenly treats the lactone mixture with excess $\ce{LiAlH4}$ in THF followed by aqueous acidic workup, resulting in a mixture of two alditols, $Y_1$ and $Y_2$ . Which of the following correctly identifies Reagent $X$ and the stereochemical nature of $Y_1$ and $Y_2$ ?

Options

A.
$X = \ce{Na-Hg}, \ce{H2O}$ (pH 3-5); $Y_1$ and $Y_2$ are diastereomers, where one is a meso compound and the other is chiral.
B.
$X = \text{DIBAL-H (1 eq)}, -78^\circ\text{C}$ ; $Y_1$ and $Y_2$ are diastereomers, and both are chiral.
C.
$X = \ce{NaBH4}, \ce{EtOH}$ ; $Y_1$ and $Y_2$ are enantiomers, and both are meso compounds.
D.
$X = \ce{H2}, \ce{Pd-BaSO4}$ ; $Y_1$ and $Y_2$ are diastereomers, and both are meso compounds.

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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

Kiliani-Fischer Chain Extension|Reduction Selectivity (Sn/HCl vs LiAlH4)

Why This Question Is Tricky

Common Mistakes

The problem tests the 'Reduction Selectivity' trap in carbohydrate chemistry.

Students often memorize $\ce{LiAlH4}$ as a universal reducer, failing to recognize that it over-reduces lactones to alditols (polyols) rather than stopping at the aldose (hemiacetal) stage.

The correct selective reagent is $\ce{Na-Hg}$ at pH 3-5.

Additionally, it tests stereochemical visualization: recognizing that the chain extension of D-lyxose yields D-galactose and D-talose, whose corresponding alditols are meso (galactitol) and chiral (talitol), respectively.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Biomolecules - Class 12
Concept	Kiliani-Fischer Chain Extension
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free