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Question #44621

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The Question

Consider the reaction of the following two bromo-ketones with 1 equivalent of $\ce{NaCN}$ in anhydrous DMF at 298 K:

Reaction 1: $\text{6-bromohexan-3-one} + \ce{NaCN} \xrightarrow{\text{DMF}} \text{Major Product } \textbf{P}$

Reaction 2: $\text{6-bromo-2,2,4,4-tetramethylhexan-3-one} + \ce{NaCN} \xrightarrow{\text{DMF}} \text{Major Product } \textbf{Q}$

Identify the correct IUPAC names for the major products $\textbf{P}$ and $\textbf{Q}$ .

Options

A.
$\textbf{P}$ is 2-ethyloxolane-2-carbonitrile; $\textbf{Q}$ is 4,4,6,6-tetramethyl-5-oxoheptanenitrile
B.
$\textbf{P}$ is 5-oxoheptanenitrile; $\textbf{Q}$ is 4,4,6,6-tetramethyl-5-oxoheptanenitrile
C.
$\textbf{P}$ is 2-ethyloxolane-2-carbonitrile; $\textbf{Q}$ is 2-(tert-butyl)-3,3-dimethyloxolane-2-carbonitrile
D.
$\textbf{P}$ is 5-oxoheptanenitrile; $\textbf{Q}$ is 2-(tert-butyl)-3,3-dimethyloxolane-2-carbonitrile

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Correct Answer

Step-by-Step Solution

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

Steric Hindrance in SN2|Nucleophilic Addition Sterics

Why This Question Is Tricky

Common Mistakes

This question tests the student's ability to unify the concept of steric hindrance across different reaction mechanisms (Nucleophilic Addition vs.

SN2).

Students often memorize that 'cyanide does SN2 on primary halides' without considering the kinetically faster carbonyl addition followed by intramolecular cyclization.

The trap lies in recognizing that while the unhindered ketone (Reaction 1) successfully forms the cyclic ether via a tetrahedral alkoxide intermediate, the highly hindered di-tert-alkyl ketone (Reaction 2) completely blocks nucleophilic addition due to severe B-strain (back-strain) in the tetrahedral transition state.

This forces the reaction exclusively down the intermolecular SN2 pathway.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Haloalkanes and Haloarenes - Class 12
Concept	Steric Hindrance in SN2
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free