Hydrogen adds to an alkyne in the presence of a metal catalyst such as palladium, platinum, or nickel in the same manner that it adds to an alkene (Section 4.11). It is difficult to stop the reaction at the alkene stage because hydrogen readily adds to alkenes in the presence of these efficient metal catalysts. The product of the hydrogenation reaction, therefore, is an alkane.
The reaction can be stopped at the alkene stage if a “poisoned” (partially deactivated) metal catalyst is used. The most commonly used partially deactivated metal catalyst is Lindlar catalyst, which is prepared by precipitating palladium on calcium carbonate and treating it with lead(II) acetate and quinoline. This treatment modifies the surface of the 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, only syn addition of hydrogen occurs (Section 5.19). Syn addition of hydrogen to an internal alkyne forms a cis alkene.
Internal alkynes can be converted into 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. Ammonia (bp = -33 °C) is a gas at room temperature, so it is kept in the liquid state by using a dry ice/acetone mixture (bp = -78 °C)
The first step in the mechanism of this reaction is transfer of the s orbital electron from sodium (or lithium) to an sp carbon to form a radical anion—a species with a negative charge and an unpaired electron. (Recall that sodium and lithium have a strong tendency to lose the single electron in their outer-shell s orbital; Section 1.3.) The radical anion is such a strong base that it can remove a proton from ammonia. This results in the formation of a vinylic radical—the unpaired electron is on a vinylic carbon. Another single-electron transfer from sodium (or lithium) to the vinylic radical forms a vinylic anion. The vinylic anion abstracts a proton from another molecule of ammonia. The product is the trans alkene.
The vinylic anion can have either the cis or the trans configuration. The cis and trans configurations are in equilibrium, but the equilibrium favors the more stable trans configuration because in this configuration the bulky alkyl groups are as far from each other as possible.