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

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 in