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Question #43957

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The Question

An aldopentose, D-lyxose, is subjected to Kiliani-Fischer chain extension to yield a mixture of two epimeric aldonolactones. One of these lactones, $Y$ , is isolated based on the property that its exhaustive reduction yields an optically inactive alditol. Lactone $Y$ is then subjected to two distinct reduction protocols:

\textbf{Protocol I:} Treatment with 1 equivalent of DIBAL-H in toluene at $-78^\circ\text{C}$ , followed by mild aqueous acidic workup, yielding compound $P$ .

\textbf{Protocol II:} Treatment with excess $\ce{NaBH4}$ in aqueous ethanol at $298\text{ K}$ , yielding compound $Q$ .

Which of the following statements correctly describes the properties of $P$ and $Q$ ?

Options

A.
$P$ is an optically active reducing sugar that undergoes mutarotation, whereas $Q$ is an optically inactive non-reducing sugar.
B.
Both $P$ and $Q$ are optically active compounds, but only $P$ gives a positive Tollens' test.
C.
$P$ is an optically active reducing sugar, whereas $Q$ is an optically active non-reducing sugar.
D.
$P$ is an optically inactive reducing sugar, whereas $Q$ is a meso compound that does not undergo mutarotation.

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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

Reduction Selectivity (Hydrides)|Kiliani-Fischer Chain Extension|Reducing Sugar Criteria

Why This Question Is Tricky

Common Mistakes

This question tests the chemoselectivity of reducing agents on aldonolactones and the stereochemical symmetry of carbohydrates.

The 'hidden variable' is the kinetic control of DIBAL-H at low temperatures, which halts reduction at the aldehyde (lactol) stage, preserving the reducing sugar properties.

In contrast, NaBH4 thermodynamically reduces the lactone to an alditol.

Furthermore, it tests the ability to visualize the stereocenters of D-lyxose extended to D-galactose, recognizing that its alditol (galactitol) is a meso compound due to a plane of symmetry, a classic stereochemical trap.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Reduction Selectivity (Hydrides)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free