Reaction of alkyne with a solution of an alkali metal in liquid ammonia gives: 

CONCEPT:

Reaction of alkyne with alkali metal in liquid ammonia

• The reaction of an alkyne with an alkali metal in liquid ammonia leads to the reduction of the alkyne.
• This reduction specifically results in the formation of a trans alkene.
• The reaction is known as the Birch reduction.
• In this reaction:
  • Liquid ammonia acts as the solvent and provides protons.
  • Alkali metal (e.g., sodium or lithium) provides electrons, which reduce the alkyne.

EXPLANATION:

• When an alkyne reacts with an alkali metal in liquid ammonia:
  • The alkali metal donates electrons to the triple bond of the alkyne, breaking it into a double bond.
  • Subsequent protonation (addition of protons from ammonia) leads to the formation of a trans alkene.
• The reaction proceeds in a stereoselective manner, favoring the trans configuration over the cis configuration.
• This is because the trans alkene is more stable due to minimized steric hindrance between the substituents.

Therefore, the correct answer is Trans alkene.