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Question #44642

Difficulty: 8/10 | 🟢 Free

The Question

An optically pure sample of $(S)$ -2-bromobutane is treated with sodium metal in dry ether at $298\text{ K}$ . The resulting coupling product mixture is subsequently subjected to careful fractional distillation. Which of the following statements correctly describes the stereochemical composition of the final alkane product mixture and the number of fractions obtained?

Options

A.
The mixture contains only the $(3S,4S)$ enantiomer due to retention of configuration; 1 fraction is obtained.
B.
The mixture contains a racemic pair and a meso compound due to a planar radical intermediate; 2 fractions are obtained.
C.
The mixture contains only the meso compound due to stereospecific facial coupling; 1 fraction is obtained.
D.
The mixture contains a racemic pair and a meso compound; 3 fractions are obtained due to different boiling points of all stereoisomers.

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

Wurtz and Wurtz-Fittig Coupling|Optical Activity and Enantiomers|Optical Isomerism Logic

Why This Question Is Tricky

Common Mistakes

The core trap tests the misconception that Wurtz coupling proceeds via an $S_N2$ -like carbanion mechanism with stereochemical inversion or retention.

Students must recognize that the reaction proceeds via a planar free radical intermediate, leading to a complete loss of optical activity and the formation of a statistical mixture of enantiomers and a meso compound.

Additionally, it tests the physical chemistry concept that enantiomers share identical boiling points (yielding one distillation fraction together), whereas diastereomers have distinct boiling points, resulting in a total of two distillation fractions from three stereoisomers.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Wurtz and Wurtz-Fittig Coupling
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free