Overview

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.

Procedure

Alkynes can be reduced to trans alkenes using alkali metals, like sodium or lithium, in liquid ammonia at low temperatures via a reaction called dissolving-metal reduction.

When alkali metals dissolve in liquid ammonia, they lose their single valence electron to the solvent, producing solvated electrons and turning the solution blue at low to moderate concentrations.

Solvated electrons act as strong reducing agents and readily add to the alkyne triple bond.

The mechanism begins with the addition of a solvated electron to the alkyne, forming a vinylic radical anion.

Note that the movement of a single electron is depicted by a single-headed curved arrow, often called a fishhook arrow. In contrast, a double-headed curved arrow signifies the movement of two electrons.

The radical anion formed in the first step is an intermediate. Here, the negative charge associated with the lone pair makes it an anion, while the unpaired electron lends a radical character.

Further, the radical anion can adopt two different configurations where the paired and unpaired electrons are oriented cis or trans to each other. The trans configuration is favored since it minimizes electronic repulsions, which influences the stereochemistry of the final product.

Next, the vinylic radical anion, a strong base, abstracts a proton from ammonia, forming a stable trans vinylic radical.

The addition of another electron to the vinylic radical gives a trans vinylic anion, which upon protonation by ammonia generates the trans alkene.

The reaction consistently involves anti addition of two hydrogen atoms across the carbon–carbon triple bond and thus is stereospecific.

However, while the dissolving-metal reduction method generates a trans alkene, reduction in the presence of Lindlar's catalyst yields a cis alkene.