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.
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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.
Mechanism:
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 PI bond (syn addition).
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{ 3 comments… read them below or add one }
nice work
if there are more than one multiple bonds in a structure, how do you know which one undergoes the reaction?
Depends on the type of multiple bonds. For alkenes, the order of reactivity is that the more alkyl groups are attached to the alkene, the more reactive it will be towards hydrogenation. If you have electron withdrawing groups attached to the alkene, they will also be less reactive towards hydrogenation.
Essentially, the more electron-rich the alkene, the more reactive it will be towards hydrogenation.