Dissolving-metal reduction of alkynes (trans alkene): practice and mechanism
Sodium in liquid ammonia transfers single electrons. The radical anion and then the vinyl anion arrange the groups as far apart as possible: the trans alkene forms.
Example
Sodium transfers two electrons one after the other and ammonia supplies the protons. The vinyl radical anion adopts the geometry with the alkyl groups as far apart as possible, giving the trans alkene.
Mechanism step by step
- 1
Sodium gives an electron to the triple bond: a radical anion forms.
- 2
The radical anion takes a proton from NH₃; the vinyl radical adopts the trans arrangement because the groups are then furthest apart.
- 3
A second electron gives the trans vinyl anion, a second proton the trans alkene.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Lindlar hydrogenation (cis alkene) · Catalytic hydrogenation
Frequently asked questions
Why does the reaction stop at the alkene?
Isolated double bonds do not accept electrons under these conditions; only alkynes (and arenes) are electron-poor enough.
How do I tell Lindlar and Na/NH₃ apart?
Lindlar: H₂ with a catalyst, syn, cis alkene. Na/NH₃: electrons and protons one by one, trans alkene.