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Question #44933

Difficulty: 8/10 | 🟢 Free

The Question

An equimolar mixture of pure (S)-2-bromobutane and ammonia is heated in a sealed tube. Due to the +I effect, the initially formed primary amine is kinetically superior to ammonia as a nucleophile, leading to a Hofmann ammonolysis cascade. The major secondary amine \textbf{X} produced in this cascade is isolated. \textbf{X} is then subjected to nucleophilic acyl substitution with excess acetyl chloride in pyridine to yield amide \textbf{Y}. Assuming all substitutions at chiral centers proceed exclusively via the $S_N2$ mechanism, which of the following represents the correct stereochemical designation of \textbf{Y}?

Options

A.
(2R, 2'R)- $N,N$ -bis(butan-2-yl)acetamide
B.
(2S, 2'S)- $N,N$ -bis(butan-2-yl)acetamide
C.
(2R, 2'S)- $N,N$ -bis(butan-2-yl)acetamide
D.
Racemic mixture of (2R, 2'R) and (2S, 2'S) isomers

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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

Ammonolysis Complexity|Nucleophilic Acyl Substitution

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hofmann-Ammonolysis cascade' and stereochemical tracking during successive $S_N2$ reactions.

The primary trap is the assumption that the secondary amine formed will be a meso compound (2R, 2'S).

Students often fail to recognize that while the electrophile undergoes inversion (S to R), the nucleophile retains its configuration (R) because no bonds to its chiral center are broken.

Additionally, it tests the kinetic superiority of primary amines over ammonia due to the +I effect, which drives the over-alkylation cascade, and confirms that Nucleophilic Acyl Substitution does not alter the established stereocenters.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Amines - Class 12
Concept	Ammonolysis Complexity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free