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1-Ethynylcyclohex-1-ene (cyclohexene with a terminal alkynyl group at C1) is treated with: (i) NaNH2 in liquid NH3, then (ii) CH3Br, to alkylate the terminal alkyne. The resulting internal alkyne is then hydrogenated using H2 with Lindlar catalyst. What is the final product?

1. cis-1-(prop-1-en-1-yl)cyclohex-1-ene
2. trans-1-(prop-1-en-1-yl)cyclohex-1-ene
3. 1-(prop-1-yn-1-yl)cyclohex-1-ene
4. cyclohexane derivative with saturated side chain

Correct answer: cis-1-(prop-1-en-1-yl)cyclohex-1-ene

Solution

NaNH2 abstracts the terminal alkyne proton to give acetylide, which reacts with CH3Br to give 1-(prop-1-yn-1-yl)cyclohex-1-ene (internal alkyne with CH3 on the triple bond). Hydrogenation over Lindlar catalyst gives syn addition, producing the cis (Z) alkene: cis-1-(prop-1-en-1-yl)cyclohex-1-ene.

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