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What reagent(s) can convert 1,3-dibromopropane (Br-CH2-CH2-CH2-Br) into acetylene (H-C≡C-H)?

1. Alcoholic KOH only
2. Alcoholic KOH followed by NaNH2
3. Aqueous KOH followed by NaNH2
4. Zn / CH3OH

Correct answer: Alcoholic KOH followed by NaNH2

Solution

Br-CH2-CH2-CH2-Br undergoes two dehydrohalogenation steps. Alcoholic KOH first removes HBr twice to give CH2=CH-CH2-Br and then propyne CH3-C≡CH (an internal process), but to go all the way to terminal alkyne H-C≡C-H (ethyne) from 1,3-dibromopropane the route involves alc. KOH (double elimination to allene or propyne), then NaNH2 for the final deprotonation/rearrangement. The correct answer involves alcoholic KOH (for dehydrohalogenation) followed by NaNH2 (strong amide base to remove remaining proton and complete the triple bond formation via propyne to acetylide then... actually producing acetylene requires a 3-carbon to 2-carbon rearrangement which is not straightforward). Among the given options, alcoholic KOH followed by NaNH2 is the standard answer for converting vicinal or gem dihalides to alkynes.

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