Reagent Friday: Palladium on Carbon (Pd/C)
Last updated: September 16th, 2020 |
Palladium On Carbon (Pd/C)
It’s been 100 years since Paul Sabatier was awarded the 1912 Nobel prize for his discovery that hydrogen gas will add to alkenes (among other molecules, such as CO2) when both are passed over finely powdered metals. This general reaction – “hydrogenation” – by which C–C (π) bonds are broken and C–H bonds are formed, was an extremely useful discovery that is still carried out on multi-ton scale today. The main industrial use of hydrogenation is in preparation of saturated fats from unsaturated fats, which makes them less likely to spoil. What lasts longer in your fridge, butter or margarine? You can thank the hydrogenation for that. Hydrogenation is also to blame for trans fats, but that’s another story.
While finely divided nickel was originally Sabatier’s catalyst of choice, extensive experimentation with other metals have shown that palladium, platinum, rhodium, and other “late” metals are also capable of assisting this transformation. Although the first image of platinum that might come to mind is that of a gleaming metal, in practice these reactions are very surface area dependent, and work much better when the finely divided, powdered form is used. This is generally called the “black” form. I can’t find a good picture of platinum black but here is a picture of the related platinum oxide. It’s pretty far from what you might expect to see at first glance for a “shiny” element like platinum.
In practice it can be a bit of a pain to weigh out trace amounts of expensive platinum or palladium metal to run reactions on small scale, so it’s more convenient to use a prepared mixture where the metal has been absorbed on a cheap, high surface area material like charcoal. The resulting palladium on carbon (Pd/C) or platinum on carbon (Pt/C) is the bench chemists’ workhorse reagent for hydrogenation reactions.
[One note here: for our purposes, palladium on carbon (Pd/C), platinum on carbon (Pt/C) or just plain palladium (Pd) or platinum (Pt) are all equivalent to each other. That is to say that they all do exactly the same reactions. There are subtle differences in reactivity between these reagents, but we needn’t concern ourselves with that here. ]
So how does it get used? All kinds of useful ways.
Reduction Of Alkenes With Pd/C And Hydrogen (H2)
First of all as already described Pd/C will reduce alkenes to alkanes when hydrogen is also present. It’s important to note that the to hydrogens from the alkene are delivered syn (i.e. the same face of the alkene). [We also go into detail on this in the chapter on alkenes]
Reduction Of Alkynes With Pd/C And Hydrogen (H2)
Pd/C and hydrogen will reduce alkynes all the way to alkanes – that is, two equivalents of H2 are added. Contrast that to Lindlar’s catalyst, which only adds one equivalent of H2 (but also in syn fashion).
Reduction Of Other Multiple Bonds With Pd/C And Hydrogen
Pd/C and hydrogen will also reduce other multiple bonds, such as NO2 (nitro groups), CN (nitriles) and C=NR (imines).
Finally, if enough heat and pressure is added, Pd/C and hydrogen gas will also reduce aromatic groups such as benzene. Note that this reaction is considerably more difficult than reducing a “normal” double bond due to the greater stability of the aromatic benzene ring.
Reduction Of Pi Bonds With Pd-C And H2 Occurs On The Surface Of The Metal
So how does it work?
It’s important to note that Pd/C is a catalyst in all of these reactions, meaning that it accelerates the rate of reactions but is not itself consumed. That is, Pd/C is a lot like a matchmaker who brings couples together, but never gets married himself. Another analogy is to think of the surface of Pd/C as kind of like a singles bar, where hydrogen and alkenes (or other organic compounds) meet, react, and leave together. Note that the hydrogen is always delivered to the same face of the alkene (“syn” addition).
P.S. You can read about the chemistry of Pd/C and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF. The Reagents App is also available for iPhone, click on the icon below!
- The Method of Direct Hydrogenation by Catalysis
Nobel Lecture, December 11, 1912
The French chemist Paul Sabatier is considered the ‘father’ of hydrogenation, and received the Nobel Prize in 1912 for his work. This is his Nobel Lecture, describing the path to discovery and his contributions to chemistry.
- APPARATUS FOR CATALYTIC REDUCTION
Roger Adams and V. Voorhees
Org. Synth. 1928, 8, 10
This describes in detail how to build a reactor for catalytic hydrogenation. Nowadays these can be commercially purchased – the “Parr reactor” is very common.
- PALLADIUM CATALYSTS
Org. Synth. 1946, 26, 77
Detailed procedures of how to prepare palladium on barium sulfate (Pd/BaSO4), palladium chloride on carbon, and both 5% and 10% palladium on carbon (Pd/C), from Organic Syntheses, a compendium of reliable methods for the preparation of organic compounds.
- The Hydrogenation of Cyclohexenes over Platinum Oxide
James-Frederick Sauvage, Robert H. Baker, and Allen S. Hussey
Journal of the American Chemical Society 1960, 82 (23), 6090-6095
Catalytic hydrogenation usually proceeds with addition of both hydrogen atoms to the same face of the double bond (syn addition). Adsorption to the catalyst surface normally involves the less sterically hindered face of the double bond, and this is seen in this paper.
- The Stereochemistry of the Hydrogenation of Cycloolefins on Supported Palladium Catalysts
Samuel Siegel and Gerard V. SmithJournal of the American Chemical Society 1960 82 (23), 6087-6090
One side-reaction that occasionally occurs with Pd-C is isomerization of adjacent stereocenters, as seen in this paper. Isomerization is less likely when using platinum as the catalyst.
- The Reaction of Sodium Borohydride with Nickel Acetate in Ethanol Solution–A Highly Selective Nickel Hydrogenation Catalyst
Herbert C. Brown and Charles A. Brown
Journal of the American Chemical Society 1963, 85 (7), 1005-1006
Catalytic hydrogenations are usually very clean reactions with little byproduct formation, but careful studies reveal that sometimes double bond migration can take place in competition with reduction. In this case, hydrogenation of 1-pentene over ‘P-2 nickel boride’ (nickel acetate reduced with NaBH4) is accompanied by some isomerization to both E– and Z-2-pentene.
- Low-temperature hydrogenation over borohydride-reduced catalysts. A new convenient procedure for improving the selectivity of reduction
Charles Allan Brown
Journal of the American Chemical Society 1969, 91 (21), 5901-5902
This paper demonstrates that 1,2-dimethylcyclohexene can be reduced with a preferential syn addition from the less hindered side over Pt under H2.
- Stereochemical Control of Reductions. 9. Haptophilicity Studies with 1,1-Disubstituted 2-Methyleneacenaphthenes
Hugh W. Thompson and Shaker Y. Rashid
The Journal of Organic Chemistry 2002, 67 (9), 2813-2825
Functional groups can also exert directing effects in catalytic hydrogenation. The substituent can interact with the catalyst surface and direct hydrogenation towards the same side that is closest to it.