Hydrogenation of Alkynes to Alkanes using Pd/C
Description: Palladium on carbon (Pd/C) catalyzes the addition of hydrogen to C–C multiple bonds. For alkynes, this reaction will occur twice to give alkanes.
Notes: Platinum (Pt) is also a good catalyst for this reaction. Note that reaction can be halted at the alkene stage if one uses Lindlar’s catalyst or Na/NH3. Delivery of the hydrogens occurs syn to the alkene, but since the final product is a linear alkane, this is of no consequence.
Notes: Note that platinum (Pt) does the exact same reaction as Pd/C
Mechanism: The detailed mechanism of this transformation is generally not considered “important” for the purposes of Org 1 / Org 2 although it is important to know that the reaction occurs on the surface of palladium metal. Both the alkyne and hydrogen are adsorbed onto the surface of the metal, and the hydrogen atoms are delivered to the same face of the π bond (syn addition).
Note: For hydrogenation of alkenes, the general trend of reactivity follows this pattern. Increasing substitution on the alkene leads to slower reaction, due to greater inaccessibility of the alkene to the surface of the Pd catalyst (steric hindrance, in other words). Thank you to Daniel in the comments for correcting an earlier error of mine .