Alkyne Reactions: Alkyne Reduction using Na and NH3

Sodium (Na) in solution with ammonia (NH3) will react with alkynes to form alkenes:

This reaction adds a hydrogen to each carbon on the alkyne bond, reducing the bond to an alkene bond. The hydrogens are added in a trans fashion, not cis :

Trans Addition

NOT cis Addition

The reaction mechanism is depicted below:

In the first step, the free electron from elemental sodium (Na) reacts with one of the carbon atoms on the alkyne bond, causing the bond to split and send each carbon atom 1 electron.

In the second step, the radical anion that was formed attacks the proton on NH3, causing the resulting NH2- to bind to Na+ and leave the reaction (not pictured).

In the third step, the free electron from a new elemental sodium (Na) reacts with free electron from the other carbon atom, forming a new radical anion.

In the fourth step, the radical anion attacks the proton on a second NH3, causing the resulting NH2- to bind to Na+ and leave the reaction (not pictured).

The reagent used for this reaction is Na or Li, and NH3.