Alkyne reduction: Lindlar (cis) vs Na/NH₃ (trans) template

But-2-yne gives (Z)-but-2-ene with H₂ over Lindlar’s catalyst, and (E)-but-2-ene with Na in liquid NH₃: electron transfer gives a radical anion, which NH₃ protonates; a second electron and proton give the trans alkene.

Mechanism step by step

H₂ on a poisoned Pd catalyst adds syn; dissolving sodium adds two electrons and two protons, anti

1. Lindlar: syn addition – H₂ on Pd/CaCO₃ poisoned with Pb(OAc)₂ and quinoline: both H add to one face; the poison stops at the alkene
2. Electron transfer – Na gives one electron to the π* orbital: a radical anion, with its two lobes trans to keep apart
3. Protonation – NH₃ protonates the carbanion carbon: a trans vinyl radical
4. Second electron – A second Na gives the vinyl anion, still trans
5. Protonation – NH₃ delivers the second H: (E)-but-2-ene

Key points

• Lindlar’s catalyst (Pd on CaCO₃ or BaSO₄, poisoned with Pb(OAc)₂ and quinoline) adds H₂ syn and is too weak to reduce the alkene: internal alkynes give cis (Z) alkenes.
• Na or Li in liquid NH₃ (−33 °C) adds an electron, a proton, an electron and a proton; the vinyl radical and anion take the less hindered trans shape, so the product is the trans (E) alkene.
• Per alkyne: 2 Na + 2 NH₃ → alkene + 2 NaNH₂. Terminal alkynes are deprotonated (NaNH₂) instead, so this works best on internal alkynes.
• H₂ with ordinary Pd/C or Pt reduces the alkyne right through to the alkane.

Drawn with 58 atoms, 14 arrows, 7 shapes – every one editable in MolDraw.

Sources

• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 9 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 23 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 15 Textbook