Reduction of Alkynes

Alkynes can be reduced to alkenes or alkanes, offering a range of synthetic possibilities. The specific outcome—whether an alkene or an alkane is formed—depends on the reagents and reaction conditions employed. We will explore several methods for achieving these reductions, highlighting the reagents used and the resulting stereochemistry.

Hydrogenation to Alkanes (Complete Reduction)

Alkynes can be fully reduced to alkanes by hydrogenation and occurs step wise. This transformation involves the addition of two moles of hydrogen (H₂) across the triple bond.

• Reagents: Excess hydrogen gas (H₂) and a standard metal catalyst, such as platinum (Pt), palladium (Pd), or nickel (Ni), are used.
• Mechanism: Two consecutive additions of hydrogen occur. The first addition converts the alkyne to an alkene, and the second addition converts the alkene to an alkane.
• Stereochemistry: The first hydrogenation (alkyne to alkene) is usually syn. The second hydrogenation can occur via either syn or anti addition. The overall stereochemistry usually isn't of concern, since the final product is an alkane, and rotation about single bonds will occur rapidly.
• Thermodynamics: Hydrogenation reactions are exothermic, meaning they release heat. The high energy of the pi bonds in the alkyne makes the overall process of complete hydrogenation to an alkane highly thermodynamically favorable. However, the first hydrogenation step (alkyne to alkene) is typically more exothermic than the second step (alkene to alkane). This difference in hydrogenation energies is due to the fact that the pi bonds in alkynes are generally weaker than the pi bonds in alkenes.
• Example: The reduction of propyne with excess H₂ and Pt will produce propane.

Hydrogenation to Alkenes (Partial Reduction)

• Reagents: The most common method uses a Lindlar catalyst. This is a "poisoned" or deactivated palladium catalyst, typically supported on calcium carbonate (CaCO₃) and "poisoned" with lead acetate (Pb(OAc)₂) or quinoline. Hydrogen gas (H₂) is also required.
• Mechanism: The Lindlar catalyst facilitates the syn addition of hydrogen across the triple bond. This means that the two hydrogen atoms add to the same side of the molecule, resulting in a cis alkene.
• Stereochemistry: The syn addition leads specifically to the cis (or Z) isomer of the alkene.
• Example: The reduction of 2-butyne with H₂ and Lindlar catalyst will produce cis-2-butene.
• Other Catalysts: Other catalysts can be used for partial hydrogenation, but they are not as common as the Lindlar catalyst due to the possibility of over-reduction to the alkane.

Reduction with Alkali Metals in Liquid Ammonia (or an amine) to form trans-alkenes

• Reagents: An alkali metal (like sodium, Na, or lithium, Li) in liquid ammonia (NH₃) or an amine (like ethylamine, EtNH₂) is used.
• Mechanism: This reaction involves a series of single electron transfers. The net addition of hydrogen atoms across the triple bond is anti.
• Stereochemistry: This reduction gives the trans (or E) alkene as the major product.
• Example: The reduction of 2-butyne with Na in liquid NH₃ will produce trans-2-butene.

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