Q5(d). Na/liquid NH₃ reduction of oct-4-yne

Introduction. Sodium in liquid ammonia gives blue solvated electrons. A dialkylalkyne takes up two electrons and two protons, one at a time, and is reduced to the trans-alkene: oct-4-yne gives (E)-oct-4-ene. This dissolving-metal reduction complements catalytic hydrogenation over Lindlar catalyst, which gives the cis-alkene.

Scheme: Dissolving metal: anti addition of 2 H. Lindlar catalyst: syn addition.

Figure: Electron, proton, electron, proton: the geometry is fixed at the vinyl radical and vinyl anion stages.

1. Electron transfer: Na → Na⁺ + e⁻(NH₃). The electron enters a π* orbital of the alkyne: radical anion.
2. Protonation: the carbanion end takes H⁺ from NH₃ (or an added alcohol): a vinyl radical. The vinyl radical inverts easily, and the trans form, with the two propyl groups apart, is more stable.
3. Second electron: the vinyl radical is reduced to a vinyl anion, which keeps the trans geometry (vinyl anions invert slowly).
4. Second protonation gives (E)-oct-4-ene. The two H atoms are added from opposite sides: net anti addition.
5. The reaction stops at the alkene: an isolated C=C has a π* orbital too high to accept an electron from Na/NH₃.

Conclusion. Na/liquid NH₃ reduces oct-4-yne by successive electron and proton transfers through trans vinyl radical and anion intermediates: (E)-oct-4-ene by anti addition, the stereochemical complement of Lindlar hydrogenation.

Application. Na/NH₃ reduction of alkynes gives trans-alkenes in pheromone synthesis; the related Birch reduction converts arenes into 1,4-dienes (used in making oral contraceptive steroids).