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Question #44023

Difficulty: 8/10 | 🟢 Free

The Question

An optically active carboxylic acid, (S)-2-phenylbutanoic acid (\textbf{A}), is subjected to two distinct reaction sequences:

\textbf{Sequence 1:} \textbf{A} is treated with 2.2 equivalents of Lithium diisopropylamide (LDA) in THF at $-78^{\circ}\text{C}$ . After 1 hour, 1.1 equivalents of methyl iodide (\ce{CH3I}) are added. The reaction is allowed to warm to room temperature, followed by standard acidic aqueous workup to yield major product \textbf{P}.

\textbf{Sequence 2:} \textbf{A} is treated with 1.0 equivalent of acetyl chloride (\ce{CH3COCl}) and 1.1 equivalents of anhydrous \ce{AlCl3} in \ce{CH2Cl2} at $25^{\circ}\text{C}$ . After 4 hours, the mixture is quenched with cold dilute HCl and extracted to yield major organic product \textbf{Q}.

Identify the correct statement regarding the major isolated organic products \textbf{P} and \textbf{Q}.

Options

A.
\textbf{P} is racemic 2-methyl-2-phenylbutanoic acid; \textbf{Q} is (S)-2-phenylbutanoic acid.
B.
\textbf{P} is (S)-2-methyl-2-phenylbutanoic acid; \textbf{Q} is racemic 2-(4-acetylphenyl)butanoic acid.
C.
\textbf{P} is racemic 2-methyl-2-phenylbutanoic acid; \textbf{Q} is (S)-2-(4-acetylphenyl)butanoic acid.
D.
\textbf{P} is methyl (S)-2-phenylbutanoate; \textbf{Q} is (S)-2-phenylbutanoic acid.

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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

Friedel-Crafts on Carboxylic Acids|Enolate Regioselectivity|Alpha-Hydrogen Acidity

Why This Question Is Tricky

Common Mistakes

The question tests the 'Stoichiometric Trap' in Friedel-Crafts reactions and the stereochemical consequences of enolate formation.

A common student error is assuming that 1 equivalent of a Lewis acid catalyst is sufficient for aromatic acylation regardless of the substrate.

However, Lewis basic functional groups (like carboxylic acids) sequester \ce{AlCl3} in a 1:1 complex, strongly deactivating the ring and requiring $>2$ equivalents of \ce{AlCl3} for the reaction to proceed.

By providing only 1.1 equivalents, the reaction is intentionally stalled, leading to the recovery of the starting material.

Additionally, it tests the fundamental concept that $\alpha$ -deprotonation to an enolate creates a planar intermediate, resulting in racemization upon subsequent alkylation.

Quick Facts About This Question

Question Information

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