Alkynes, those triple-bonded hydrocarbons, can be transformed into alkenes or alkanes through reduction. This process involves adding hydrogen atoms to break down the triple bond, with different methods yielding different products.

The choice of catalyst or reducing agent determines the outcome. Palladium catalysts can give you alkanes or cis-alkenes, while sodium in liquid ammonia produces trans-alkenes. Understanding these reactions is key to controlling organic synthesis.

Reduction of Alkynes

Process of alkyne reduction

• Catalytic hydrogenation
  • Palladium on carbon (Pd/C) catalyst reduces alkynes to alkanes by adding two equivalents of hydrogen ($\ce{H2}$)
  • Lindlar catalyst (Pd on $\ce{CaCO3}$, poisoned with lead acetate and quinoline) partially reduces alkynes to cis-alkenes by adding one equivalent of hydrogen ($\ce{H2}$)
    • Catalytic poisoning prevents over-reduction to alkanes
• Dissolving metal reduction
  • Sodium ($\ce{Na}$) or lithium ($\ce{Li}$) in liquid ammonia ($\ce{NH3}$) reduces alkynes to trans-alkenes by adding one equivalent of hydrogen ($\ce{H2}$)

Stereochemistry of reduction methods

• Lindlar catalyst reduction
  • Yields cis-alkenes as the major product through a stereospecific reaction where hydrogen adds to the same side of the alkyne
• Sodium/lithium in liquid ammonia reduction
  • Yields trans-alkenes as the major product through a stereospecific reaction where hydrogen adds to opposite sides of the alkyne

Mechanism of alkali metal reduction

1. Electron transfer from alkali metal to alkyne forms a radical anion intermediate
2. Protonation of the radical anion by ammonia yields a vinyl radical
3. Second electron transfer from alkali metal to vinyl radical forms a vinyl anion
4. Final protonation of the vinyl anion by ammonia yields the trans-alkene product

• Stereochemistry favors trans-alkene formation due to the stability of the vinyl anion intermediate which adopts a linear geometry to minimize repulsion between the negative charge and the alkyl substituents

Additional Reduction Methods and Considerations

• Metal hydrides (e.g., LiAlH4) can be used for selective reduction of alkynes
• Reaction kinetics influence the rate and selectivity of alkyne reduction
• Hydrogenolysis can occur as a side reaction in some reduction processes