The Addition of Hydrogen to an Alkyne

Alkynes can be reduced by catalytic hydrogenation just as alkenes can (Section 6.12). The initial product of hydrogenation is an alkene, but it is difficult to stop the reaction at this stage because of hydrogen s strong tendency to add to alkenes in the presence of these efficient metal catalysts. The final product of the hydrogenation reaction, therefore, is an alkane. 

Lindlar catalyst is prepared by precipitating palladium on calcium carbonate and treating it with lead(ll) acetate and quinoline. This treatment modifies the surface of palladium, making it much more effective at catalyzing the addition of hydrogen to a triple bond than to a double bond. 

Because the alkyne sits on the surface of the metal catalyst and the hydrogens are delivered to the triple bond from the surface of the catalyst, both hydrogens are delivered to the same side of the double bond. In other words, syn addition of hydrogen occurs (Section 6.15). Syn addition of H2 to an internal alkyne forms a cis alkene. 

Internal alkynes can be converted to trans alkenes using sodium (or lithium) in liquid ammonia. The reaction stops at the alkene stage because sodium (or lithium) reacts more rapidly with triple bonds than with double bonds. This reaction is called a dissolving metal reduction. Ammonia is a gas at room temperature (bp = — 33 °C), so it is kept in the liquid state by cooling the reaction flask in a dry ice/acetone mixture, which has a temperature of -78 °C. 

As sodium dissolves in liquid ammonia, it forms a deep blue solution of dissolved Na and electrons. 

---