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Question #43983

Difficulty: 8/10 | 🟢 Free

The Question

Consider the optically active ester, ( $S$ )-2,4-dinitrophenyl 2-phenylpropanoate (Compound \textbf{X}). It is subjected to ammonolysis using isopropylamine ( $iPrNH_2$ ) under two different solvent conditions at 298 K:

Reaction 1: \textbf{X} + excess $iPrNH_2$ in anhydrous $CH_3CN$ $\longrightarrow$ Product \textbf{P}

Reaction 2: \textbf{X} + excess $iPrNH_2$ in $H_2O$ /EtOH mixture $\longrightarrow$ Product \textbf{Q}

Assuming no other side reactions, identify the stereochemical nature of the major amide products \textbf{P} and \textbf{Q}.

Options

A.
\textbf{P} is racemic $N$ -isopropyl-2-phenylpropanamide; \textbf{Q} is ( $S$ )- $N$ -isopropyl-2-phenylpropanamide.
B.
\textbf{P} is ( $S$ )- $N$ -isopropyl-2-phenylpropanamide; \textbf{Q} is racemic $N$ -isopropyl-2-phenylpropanamide.
C.
Both \textbf{P} and \textbf{Q} are ( $S$ )- $N$ -isopropyl-2-phenylpropanamide.
D.
\textbf{P} is ( $R$ )- $N$ -isopropyl-2-phenylpropanamide; \textbf{Q} is ( $S$ )- $N$ -isopropyl-2-phenylpropanamide.

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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

Alpha-Hydrogen Acidity|Ammonolysis Complexity|Basicity-Solvation Competition

Why This Question Is Tricky

Common Mistakes

The question tests the competition between nucleophilic acyl substitution ( $B_{Ac}2$ ) and the E1cB mechanism in ester aminolysis.

Students typically assume amines always act as nucleophiles attacking the carbonyl carbon.

The 'hidden variable' is the solvent's role in stabilizing the transition state.

Protic solvents stabilize the tetrahedral intermediate via H-bonding, favoring $B_{Ac}2$ (retention of stereochemistry).

Aprotic solvents fail to stabilize this intermediate, allowing the amine to act as a base, abstracting the acidic alpha-proton to form a ketene intermediate (E1cB), which leads to racemization.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Aldehydes, Ketones, and Carboxylic Acids - Class 12
Concept	Alpha-Hydrogen Acidity
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free