JEE Challenger
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ChemMedAdv 2014 • Paper 2

Identify Major Product in Intramolecular Cyclization via Grignard Addition

Determine the major organic product formed in the following reaction sequence:

Question Diagram 1

Options

A
Option A Diagram 1
B
Option B Diagram 1
C
Option C Diagram 1
D
Option D Diagram 1
Correct

Topics & Concepts

Haloalkanes and HaloarenesGrignard ReagentNucleophilic Substitution

Step-by-Step Solution

To determine the major organic product of the reaction, let us break down the reaction mechanism step-by-step:

Step 1: Nucleophilic Addition of the Grignard Reagent

The starting material is 5-chloropentan-2-one (Cl−CH2−CH2−CH2−C(=O)CH3\text{Cl}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{C}(=\text{O})\text{CH}_3).

When treated with methylmagnesium bromide (CH3MgBr\text{CH}_3\text{MgBr}) in dry ether at 0 ∘C0\,^\circ\text{C}, the nucleophilic methyl carbanion (CH3−\text{CH}_3^-) attacks the electrophilic carbonyl carbon (C−2\text{C}-2):

Cl−CH2−CH2−CH2−CO∥−CH3+CH3MgBr⟶Cl−CH2−CH2−CH2−C∣(CH3)2O−MgBr+∣\text{Cl}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\overset{\underset{\parallel}{\text{O}}}{\text{C}}-\text{CH}_3 + \text{CH}_3\text{MgBr} \longrightarrow \text{Cl}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\overset{\underset{\vert}{\text{O}^-\text{MgBr}^+}}{\underset{\vert}{\text{C}}(\text{CH}_3)_2}

This forms a magnesium alkoxide intermediate containing a primary alkyl chloride tail.

Step 2: Intramolecular SN2S_N2 Cyclization

The alkoxide oxygen (O−\text{O}^-) acts as a strong internal nucleophile. It performs an intramolecular nucleophilic substitution (SN2S_N2) reaction by attacking the primary carbon (C−5\text{C}-5) bearing the chlorine leaving group (Cl−\text{Cl}^-):

Cl−CH2−CH2−CH2−C(O−)(CH3)2⟶2,2-dimethyltetrahydrofuran+Cl−\text{Cl}-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{C}(\text{O}^-)(\text{CH}_3)_2 \longrightarrow \text{2,2-dimethyltetrahydrofuran} + \text{Cl}^-

Counting the atoms involved in the ring formation: Oxygen→C−2→C−3→C−4→C−5\text{Oxygen} \rightarrow \text{C}-2 \rightarrow \text{C}-3 \rightarrow \text{C}-4 \rightarrow \text{C}-5

This forms a highly stable 5-membered cyclic ether (a substituted tetrahydrofuran ring) with two methyl groups (−CH3-\text{CH}_3) attached to the carbon adjacent to the oxygen atom (C−2\text{C}-2).

Step 3: Acidic Workup

Subsequent treatment with aqueous acid (H3O+\text{H}_3\text{O}^+) hydrolyzes any remaining magnesium salts, yielding 2,2-dimethyltetrahydrofuran as the final major product.

This corresponds to the structure shown in Option (D).

Correct Answer: (D)

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