Question

When butyne-2 is reacted with Na in liquid ammonia in presence of $C_2H_2OH$ then the major product formed is

• A. Cis-but-2-ene
• B. Trans-but-2-ene
• C. But-1-ene
• D. No reaction

Answer 1

Answer: (B)

Trans-but-2-ene

Answer 2

The reaction of butyne-2 (an internal alkyne) with sodium (Na) in liquid ammonia ($NH_3(l)$) in the presence of a proton source like ethanol ($C_2H_2OH$) is known as the Birch reduction. When an internal alkyne undergoes Birch reduction, it is selectively reduced to a trans-alkene. The stereochemistry of the resulting alkene is predominantly trans because the intermediate radical anion and vinyl radical prefer a trans configuration to minimize steric repulsion between the alkyl groups.