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Question #43980

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following three reaction sequences performed on 2-methylcyclohexan-1-one:

\textbf{Sequence I:}

\ce{LDA} (1.1 equiv), \ce{THF}, $-78^\circ\text{C}$
Allyl bromide

\textbf{Sequence II:}

Pyrrolidine, cat. $p$ -\ce{TsOH}, Benzene, $\Delta$ (Dean-Stark)
Allyl bromide
\ce{H3O+}

\textbf{Sequence III:}

\ce{TMSCl}, \ce{Et3N}, \ce{DMF}, $130^\circ\text{C}$
\ce{MeLi}, \ce{THF}, $0^\circ\text{C}$
Allyl bromide

Let the major organic products of Sequences I, II, and III be X, Y, and Z respectively. Identify X, Y, and Z.

Options

A.
X = 2-allyl-6-methylcyclohexan-1-one; Y = 2-allyl-6-methylcyclohexan-1-one; Z = 2-allyl-2-methylcyclohexan-1-one
B.
X = 2-allyl-6-methylcyclohexan-1-one; Y = 2-allyl-2-methylcyclohexan-1-one; Z = 2-allyl-2-methylcyclohexan-1-one
C.
X = 2-allyl-2-methylcyclohexan-1-one; Y = 2-allyl-6-methylcyclohexan-1-one; Z = 2-allyl-6-methylcyclohexan-1-one
D.
X = 2-allyl-6-methylcyclohexan-1-one; Y = 2-allyl-2-methylcyclohexan-1-one; Z = 2-allyl-6-methylcyclohexan-1-one

Check Your Answer

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

Alpha-Hydrogen Acidity|Enolate Regioselectivity

Why This Question Is Tricky

Common Mistakes

This question tests the distinction between kinetic and thermodynamic control in enolate/enamine alkylation, focusing on a critical exception.

Students typically memorize that enamines are thermodynamic intermediates and assume they form at the more substituted $\alpha$ -carbon (leading to Option B).

However, they overlook the severe $A^{1,3}$ (allylic) strain between the pyrrolidine ring and the $\alpha$ -methyl group, which forces the enamine to form at the less substituted carbon.

This makes the enamine alkylation product identical to the kinetic enolate alkylation product, serving as a lethal trap for rote learners.

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