Partial Reduction of Alkynes with Na/NH₃
Partial Reduction of Alkynes with Na/NH₃: The Na/NH₃ reduction converts an alkyne into an alkene without reducing it all the way to an alkane.
What happens
The Na/NH₃ reduction converts an alkyne into an alkene without reducing it all the way to an alkane. Only one π bond is removed (C≡C → C=C). The reaction is a dissolving-metal reduction, and for an internal alkyne it gives the trans (E) alkene.
Reaction summary
Alkyne + Na/NH₃(l) → trans (E) alkene (C≡C → C=C, anti addition)
Reagents and conditions
- Starting material: 2-Butyne (internal alkyne)
- Reagents: Na, NH₃(l)
- Products: (E)-2-Butene (trans alkene)
Selectivity
- Stereoselectivity: Anti addition: the two new hydrogens are added from opposite sides of the alkyne, so a suitable internal alkyne gives the trans (E) alkene. This is the most important part of the reaction.
Common mistakes
- It is a partial reduction because the reaction stops at the alkene. It does NOT continue C≡C → C=C → C–C. That makes it useful when you want to reduce an alkyne but keep a double bond.
Memory trick
Na/NH₃ = NO HUGGING. The two hydrogens stay on opposite sides: ANTI → TRANS. So: Na/NH₃ → TRANS.