JEE Advanced 2014 · Paper 2 · Chemistry Q23

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ChemistrySingle correct+3 / -1Question 23 of 60

The major product in the following reaction is

5-Chloropentan-2-one, Cl-CH₂CH₂CH₂-CO-CH₃, treated with 1. CH₃MgBr, dry ether, 0 °C; 2. aq. acid.

2-Methylpent-4-en-2-ol, H₂C=CH-CH₂-C(OH)(CH₃)₂.

2-Methylenetetrahydrofuran (a five-membered cyclic ether with an exocyclic =CH₂ on the carbon next to O).

2,2-Dimethyltetrahydrofuran (a five-membered cyclic ether with two CH₃ groups on the carbon next to O).

Answer and worked solution

Official answer: D

Step 1. $\ce{CH3MgBr}$ adds to the ketone at 0 °C, giving the tertiary alkoxide $\ce{Cl-CH2CH2CH2-C(CH3)2-O^-}$ .

Step 2. The alkoxide is positioned to attack the carbon bearing Cl four atoms away. Intramolecular $\mathrm S_\mathrm N2$ closes a five-membered ring, displacing chloride.

Product. 2,2-Dimethyltetrahydrofuran. (The aqueous acid work-up does not open the ether.)

(D)

Topics:Nucleophilic addition Ethers SN1 and SN2

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JEE Advanced 2014 · Paper 2, Chemistry question 23