Alkyne Reactions: Partial Hydrogenation to cis‑Alkenes with H₂ / Lindlar or Ni₂B (P‑2)

Lindlar’s catalyst—palladium deposited on CaCO₃ or BaSO₄ and “poisoned” with additives such as Pb(OAc)₂/PbCO₃ and quinoline—selectively reduces alkynes to cis (Z) alkenes. On the attenuated surface, H₂ dissociates to Pd–H (or Ni–H), the alkyne π system adsorbs to the same face, and a single syn hydrogenation step delivers the (Z)-alkene, which desorbs before another hydrogenation can occur. Terminal alkynes stop at terminal alkenes, while further reduction to alkanes is strongly suppressed. Need the trans (E) alkene instead? Switch to a dissolving-metal reduction (Na/NH₃(l)). Nickel boride (Ni₂B; P‑2 nickel) with H₂ is a functionally equivalent semihydrogenation catalyst: the alkyne π-binds to a deactivated Ni surface, one syn delivery of H₂ furnishes the cis (Z) alkene, and desorption suppresses over-reduction. Use Ni₂B interchangeably with Lindlar at the undergraduate level.

This guide walks through the surface mechanism using but-1-ynylcyclohexane, highlights why Lindlar catalysts stop at the cis alkene, and provides practical tips, worked examples, and solver tool integrations. For a full reduction contrast, see the alkyne hydrogenation (H₂) guide.

Quick Summary

• Reagents/conditions: H₂ (balloon to low pressure) with Lindlar catalyst (Pd/CaCO₃ or Pd/BaSO₄, poisoned with Pb(OAc)₂ or PbCO₃ and quinoline) or Ni₂B (P‑2 nickel boride); solvents such as EtOH, EtOAc, or hexanes; 0 °C → room temperature.
• Outcome: Each C≡C is reduced to a cis (Z) alkene by a single syn delivery of H₂. Terminal alkynes give terminal alkenes (no E/Z issue).
• Selectivity: Surface poisoning prevents further hydrogenation of the newly formed alkene; arenes and isolated C=C bonds remain untouched.
• Stereochemistry: Syn addition on the metal surface enforces the cis geometry.
• Contrast: Unpoisoned Pd/C or Pt/C continues to the alkane; Na/NH₃(l) gives trans (E) alkenes via dissolving-metal conditions; H₂/Ni₂B behaves like Lindlar (cis, single syn addition).

Mechanism (4 Surface Steps) (identical for H₂/Lindlar and H₂/Ni₂B—substitute Ni–H for Pd–H in each frame)

Showcased substrate: but-1-ynylcyclohexane → [(Z)-but-1-enyl]cyclohexane. Illustrations generated with the Lindlar RDKit mechanism.

Hydrogen gas adsorbs and splits into adjacent Pd–H (or Ni–H) fragments. Catalyst poisoning (Pb salts, quinoline) or Ni₂B deactivation dampens activity so the surface holds only the hydrides needed for a single syn delivery.

The alkyne aligns on the catalyst face, trans to the hydrides. A surface Pd–H (or Ni–H) migrates to the nearer carbon, producing a half-hydrogenated vinyl species still coordinated to Pd or Ni.

The remaining surface hydride transfers to the adjacent carbon, completing a single syn addition of H₂ and locking in the Z relationship between substituents.

Because the surface is poisoned or deactivated, the freshly formed alkene does not stay adsorbed long enough for another hydrogen addition. The Pd or Ni site remains active for the next alkyne.

Mechanistic Checklist (Exam Focus)

• Product: cis (Z) alkene via a single syn surface hydrogenation.
• No further reduction to alkane (contrast with unpoisoned Pd/C or Pt/C).
• Terminal alkynes → terminal alkenes (no E/Z designation required).
• Trans (E) alkenes demand dissolving-metal conditions (Na/NH₃(l)).
• Closed-shell, surface-mediated process; rearrangements are not observed.
• H₂/Ni₂B (P‑2 nickel boride) gives the same cis outcome and syn surface mechanism as Lindlar; treat them interchangeably for scope and stereochemistry.

Worked Examples

Internal alkyne → cis internal alkene via syn addition.

Aryl alkyne → cis styryl fragment; the phenyl ring locks in the syn addition.

Aryl rings remain untouched; only the C≡C is reduced.

Multiple Unsaturations & Selectivity

• Alkynes adsorb more strongly than isolated alkenes, so Lindlar conditions pick off C≡C first. Ordinary C=C bonds typically survive.
• Molecules bearing two alkynes can be partially or fully reduced depending on H₂ equivalents and reaction time; limiting hydrogen or catalyst permits mono-reduction.
• Dissolving-metal reduction (Na/NH₃(l)) is the complementary route for trans (E) alkenes.
• Ni₂B mirrors Lindlar selectivity: alkynes are reduced to cis alkenes while isolated alkenes generally persist.

Practical Tips & Pitfalls

• Catalyst handling: Pd catalysts are pyrophoric when dry; keep them moist and filter under inert atmosphere where possible.
• Ni₂B (P‑2): supplied as a moist, pyrophoric-when-dry catalyst cake; handle like Pd/C. Activity can vary by lot—monitor closely to avoid slow bleed-through to alkane under extended times or higher H₂ pressures.
• Poisoning level: Quinoline and lead salts moderate activity. Too little poison risks over-reduction; too much stalls the reaction.
• Gas transfer: Provide efficient stirring and adequate H₂ (balloon with agitation or low-pressure reactor) to avoid mass-transfer limits.
• Functional groups: Benzyl/allyl hydrogenolysis is greatly diminished vs Pd/C, but verify compatibility in multi-step sequences.
• Specify requirements: Do not expect a trans alkene; explicitly choose Na/NH₃(l) if the E isomer is desired.

Exam-Style Summary

H₂/Lindlar or H₂/Ni₂B (P‑2) converts alkynes to cis (Z) alkenes via a single syn surface hydrogenation. Catalyst poisoning suppresses the second addition, so alkane formation is avoided. Terminal alkynes become terminal alkenes; for trans outcomes use dissolving-metal reduction.

Interactive Toolbox

• Mechanism Solver — follow the Lindlar surface sequence step by step.
• Reaction Solver — visualize cis vs alkane outcomes by toggling Lindlar versus unpoisoned Pd/Pt.
• IUPAC Namer — verify names such as [(Z)-but-1-enyl]cyclohexane, (Z)-2-methylhex-3-ene, and [(Z)-prop-1-enyl]benzene.
• Select catalyst: Lindlar ⇄ Ni₂B (P‑2) (identical cis outcome).