Reduction of Alkynes to trans-Alkenes: Sodium in Liquid Ammonia

Alkynes can be reduced to trans-alkenes using sodium or lithium in liquid ammonia. The reaction, known as dissolving metal reduction, proceeds with an anti addition of hydrogen across the carbon–carbon triple bond to form the trans product. Since ammonia exists as a gas (bp = −33°C) at room temperature, the reaction is carried out at low temperatures using a mixture of dry ice (sublimes at −78°C) and acetone. 

When dissolved in liquid ammonia, an alkali metal, such as sodium, dissociates into a cation and a free electron. Ammonia molecules surround the free electrons, creating solvated electrons that impart a blue color to the solution. Solvated electrons are strong reducing agents and readily add to the alkyne triple bond.

Limitation:

The reduction of terminal alkynes with sodium in liquid ammonia does not proceed as efficiently as the reduction of internal alkynes. This is because terminal alkynes have acidic protons that readily react with the sodium–liquid ammonia mixture to form sodium acetylide. Stoichiometrically, three moles of a terminal alkyne undergo metal-dissolved reduction to give only one mole of the corresponding alkene and two moles of sodium acetylide.

Therefore, the reaction conditions need to be modified to completely convert terminal alkynes to alkenes. A common approach involves adding ammonium sulfate to the reaction mixture. The ammonium ion released into the solution protonates the acetylide, thus preserving the terminal alkyne for subsequent reduction.