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A dibromoalkane undergoes two successive elimination reactions: first with alcoholic KOH and then with NaNH2, ultimately yielding acetylene (HC≡CH). Which dibromoalkane is the starting material?

1. BrCH2CH2CH2Br (1,3-dibromopropane)
2. BrCH2CH2Br (1,2-dibromoethane)
3. BrCH2CH2CH2CH2Br (1,4-dibromobutane)
4. CH3CHBrCH2Br (1,2-dibromopropane)

Correct answer: BrCH2CH2Br (1,2-dibromoethane)

Solution

Only 1,2-dibromoethane has exactly 2 carbons and vicinal bromines that allow stepwise dehydrobromination first to vinyl bromide then to acetylene.

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