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Question #44437

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following reaction sequence: $\text{4-methylpentan-2-one} \xrightarrow[ \text{2. } \text{PhCHO, THF, } -78^\circ\text{C} \to \text{rt} ]{ \text{1. LDA (1.05 eq), THF, } -78^\circ\text{C} } \mathbf{X} \xrightarrow{ p\text{-TsOH, Benzene, } \Delta } \mathbf{P}$ Identify the major product $\mathbf{P}$ .

Options

A.
(1E)-5-methyl-1-phenylhex-1-en-3-one
B.
(3E)-3-isopropyl-4-phenylbut-3-en-2-one
C.
1-hydroxy-5-methyl-1-phenylhexan-3-one
D.
(1E)-4,4-dimethyl-1-phenylpent-1-en-3-one

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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

Nucleophilic Addition Sterics|Enolate Regioselectivity|Aldol Condensation Directionality

Why This Question Is Tricky

Common Mistakes

This problem tests the critical distinction between Kinetic and Thermodynamic enolate control, specifically focusing on the 'Steric Controlled Kinetic Enolate' exception.

Students often fall into the trap of predicting the most stable (thermodynamic) enolate, which would involve deprotonation at the more substituted C3 position to yield 3-isopropyl-4-phenylbut-3-en-2-one.

The hidden variable here is the interplay between the extreme steric bulk of the LDA base and the cryogenic temperature ( $-78^\circ\text{C}$ ).

This combination completely overrides thermodynamic stability, forcing an irreversible deprotonation at the least hindered C1 position, thereby dictating the regiochemistry of the final Aldol condensation product.

Quick Facts About This Question

Question Information

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