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JEE Main levelSingle correct optionDifficulty 2 of 3ro174

Pent-2-yne is reduced with Na in liquid to trans-pent-2-ene. The stereochemistry arises mainly because

  1. Ahydrogen is delivered to one face of the triple bond on a metal surface
  2. Bthe intermediate vinyl anion with the two alkyl groups trans to each other is more stable than the cis one
  3. Cthe cis alkene is isomerised to trans by the base afterwards
  4. Dthe radical anion is protonated twice at the same carbon

Answer: B

In the dissolving metal reduction no catalyst surface is involved. The vinyl anion formed after the first protonation can invert rapidly and prefers the geometry with the larger groups trans because of less steric and electronic repulsion. Protonation of this anion fixes the trans alkene.
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