Partial reduction [Na/NH3]
Partial Reduction (Na/NH3) Definition:
Alkynes treated with sodium in ammonia are reduced to trans alkenes. Alternatively, potassium in ammonia can also be used.
Partial Reduction (Na/NH3) Explained:
When we use sodium metal dissolved in liquid ammonia (dissolving-metal reduction) as the reagent for the reduction of alkynes, we obtain trans alkenes as the products. Unlike sodium amide in liquid ammonia, which functions as a strong base, elemental sodium in liquid ammonia acts as a powerful electron donor (reducing agent):
STEP 3. A single electron is transferred from the sodium atom to the alkenyl radical, generating an anion:
As we can see, the mechanism is comprised of the following four steps: 1) electron transfer, 2) proton transfer, 3) electron transfer, and 4) proton transfer. This is, the net addition of the molecular hydrogen is achieved via the installation of two electrons and two protons in the following order: e–, H+, e–, H+. The trans stereochemistry of the final alkene is set in the first two steps of the mechanism. Under the reaction conditions (liquid NH3, -33°C), the second one-electron transfer takes place faster than cis-trans equilibration of the radical. This type of reduction typically provides >98% stereochemically pure trans alkene.