Hydrogenation of Alkynes to Alkanes using Pd/C

by James

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.

Examples:

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).

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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 .

reactivity trends

{ 7 comments… read them below or add one }

suwedi December 12, 2011 at 2:54 pm

nice work

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Alexandria Horner November 18, 2012 at 12:00 am

if there are more than one multiple bonds in a structure, how do you know which one undergoes the reaction?

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james November 19, 2012 at 11:18 am

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. Thanks to commenter Daniel for correcting this error. See updated figure at the bottom of the page.

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Kat May 27, 2013 at 10:53 pm

Just an FYI the middle of the page you have accidentially typed in mechanisms when it should state Examples .

You are doing an amazing job thank you so much for these references!

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james May 29, 2013 at 4:47 pm

Thank you!

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Daniel August 4, 2014 at 10:13 pm

Hey everyone,

This is so wrong… see Smith, M. B. Organic Synthesis. 3 ed. pages 422-451.
More accurately in page 432, where they say that more highly substituted alkenes do not adsorb on the catalyst as effectively and are not readily reduced. So, the more substituted olefin hydrogenated slower. And if you see in Handbook of Organopalladium chemistry, there are some instances displaying that unsaturated carbonyls hydrogenation sometimes is faster than ordinary olefins.

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James August 5, 2014 at 1:06 pm

Thanks for correcting this error.

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