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Which of the following reductions with Na in liquid give the trans alkene as the major organic product?

  1. Ahex-3-yne
  2. Bhex-1-yne
  3. Cbut-2-yne
  4. Doct-4-yne

Answer: A, C, D

Internal alkynes accept two electrons and are protonated stepwise, giving the trans alkene via the more stable trans vinyl anion. This holds for hex-3-yne, but-2-yne and oct-4-yne, which give trans-hex-3-ene, trans-but-2-ene and trans-oct-4-ene. Hex-1-yne is deprotonated to the acetylide and is not reduced.
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