Question

In the following reaction, the major product (H) is

• A.
• B.
• C.
• D.

Hint:

Using a simple frame or just bolding for the box Key Points: Grignard reagents (RMgX) add to ketones/aldehydes to form alkoxides. Alkoxides are strong nucleophiles. Intramolecular S$_N$2 reactions can occur if a nucleophile and a suitable leaving group are present in the same molecule, appropriately positioned (favored for 5/6-membered rings). The sequence is: 1) Grignard addition, 2) Intramolecular cyclization.

Answer 1

The Correct Option is C

The reaction proceeds in two main steps: (A) Grignard Reaction: The nucleophilic methyl group from MeMgBr attacks the electrophilic carbonyl carbon of the ketone (5-chloro-2-pentanone). This forms a tertiary alkoxide intermediate after the C=O \(\pi\)-bond breaks.

Cl-CH₂-CH₂-CH₂-C(=O)-Me + MeMgBr → Cl-CH₂-CH₂-CH₂-C(O^-MgBr⁺)(Me)-Me (Intermediate Alkoxide)
(B) Intramolecular S_N2 Cyclization: The negatively charged oxygen (alkoxide) acts as an internal nucleophile and attacks the carbon atom bonded to the chlorine (an electrophilic site). Chlorine acts as the leaving group. This forms a stable five-membered ring (tetrahydrofuran derivative). This intramolecular reaction is faster than intermolecular reactions or protonation by trace protic sources before workup, especially for forming 5- or 6-membered rings.

Intramolecular attack leading to ring closure:
O^--CH₂-CH₂-CH₂-C(Me)(Me)-Cl → O-CH₂-CH₂-CH₂-C(Me)(Me) + Cl^-
(C) Workup: The acidic workup (H₃O⁺) neutralizes any remaining reagents and byproducts but doesn't change the already formed cyclic ether product (H). The final major product (H) is 2,2-dimethyltetrahydrofuran, which corresponds to option (C).