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Question #44490

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The Question

Consider the following reaction sequence starting from 6-methoxy-3,4-dihydronaphthalen-2(1H)-one (\textbf{SM}):

Step 1: \textbf{SM} $\xrightarrow[2. \text{ CH}_3\text{I}]{\text{1. LDA (1.05 eq), THF, } -78^\circ\text{C}}$ \textbf{P} (Major)

Step 2: \textbf{P} $\xrightarrow[2. \text{ H}_2\text{O}]{\text{1. POCl}_3\text{, DMF, } 0^\circ\text{C} \rightarrow 80^\circ\text{C}}$ \textbf{Q} (Major)

Step 3: \textbf{Q} $\xrightarrow{\text{NaOMe, MeOH, } \Delta}$ \textbf{R} (Major)

Identify the correct IUPAC names for the major products \textbf{P} and \textbf{R}.

Options

A.
\textbf{P} = 6-methoxy-3-methyl-3,4-dihydronaphthalen-2(1H)-one; \textbf{R} = 2,6-dimethoxy-3-methyl-3,4-dihydronaphthalene-1-carbaldehyde
B.
\textbf{P} = 6-methoxy-1-methyl-3,4-dihydronaphthalen-2(1H)-one; \textbf{R} = 2,6-dimethoxy-1-methyl-3,4-dihydronaphthalene-3-carbaldehyde
C.
\textbf{P} = 6-methoxy-3-methyl-3,4-dihydronaphthalen-2(1H)-one; \textbf{R} = 5-formyl-6-methoxy-3-methyl-3,4-dihydronaphthalen-2(1H)-one
D.
\textbf{P} = 6-methoxy-1-methyl-3,4-dihydronaphthalen-2(1H)-one; \textbf{R} = 7-formyl-6-methoxy-1-methyl-3,4-dihydronaphthalen-2(1H)-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

Electrophilic Substitution Orientation|Enolate Regioselectivity|Electromeric Effect

Why This Question Is Tricky

Common Mistakes

This problem tests the student's ability to navigate competing reactive sites and control mechanisms.

The first trap is the regioselectivity of enolate formation: students often mistakenly choose the benzylic C1 position for deprotonation, forgetting that LDA at low temperatures is under strict kinetic control, favoring the less hindered C3 position.

The second, more lethal trap, is the Vilsmeier-Haack reaction on a substrate with a highly activated aromatic ring.

Students will be tempted to perform Electrophilic Aromatic Substitution (EAS) at C5 or C7.

However, the enolization of the ketone provides a much more nucleophilic $\pi$ -system.

The reaction proceeds via the thermodynamic enol (conjugated with the arene) to yield a $\beta$ -chloro- $\alpha,\beta$ -unsaturated aldehyde, completely overriding EAS.

Finally, the problem tests the concept of activated nucleophilic vinylic substitution, a mechanism often overlooked compared to its aromatic counterpart.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Hydrocarbons - Class 11 Part 2
Concept	Electrophilic Substitution Orientation
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free