9.11: Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Catalytic hydrogenation reactions help evaluate the relative thermodynamic stability of hydrocarbons. For example, the heat of hydrogenation of acetylene is −176 kJ/mol, and that of ethylene is −137 kJ/mol. The higher exothermicity associated with the addition of hydrogen to acetylene suggests that it is thermodynamically less stable than ethylene.
Catalytic reduction of alkynes can be stopped at the cis-alkene stage using a modified or poisoned catalyst such as Lindlar or P-2 catalyst. The Lindlar catalyst consists of palladium metal deposited on calcium carbonate and modified using lead acetate and quinoline; the P-2 catalyst is a nickel-boride complex.
A modified catalyst lowers the activation energy for the reduction of the first π bond; however, it is not powerful enough to catalyze the reduction of the second π bond. For example, hydrogenation of 2-pentyne over Lindlar catalyst gives cis-2-pentene.
Hydroboration-protonolysis is a non-catalytic method for the conversion of internal alkynes into cis-alkenes. The reaction involves treating an internal alkyne with borane to form a trialkenylborane intermediate followed by treatment with acetic acid to yield the desired cis-alkene.