Organic Reagents

By James Ashenhurst

Reagent Friday: Palladium on Carbon (Pd/C)

Last updated: September 16th, 2020 |

Palladium On Carbon (Pd/C) 

In a blatant plug for the Reagent Guide and the Reagents App for iPhone, each Friday  I profile a different reagent that is commonly encountered in Org 1/ Org 2. 

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.

-image-of-finely-powdered-platinum-oxide

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]

palladium-on-carbon-in-the-hydrogenation-of-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).

palladium-on-carbon-in-the-hydrogenation-of-alkuynes

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).

palladium-on-carbon-reduction-of-nitro-cyano-and-imino-groups

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.

palladium-on-carbon-with-h2-reduction-of-aromatic-rings

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).

palladium-on-carbon-reduction-of-alkenes-on-the-surface-of-a-metal-mechanism

It’s also possible to substitute the heavy isomer of hydrogen (deuterium, D) for H2, which will deliver D to the alkene.

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!

 


(Advanced) References and Further Reading

  1. The Method of Direct Hydrogenation by Catalysis
    Paul Sabatier
    Nobel Lecture, December 11, 1912
    https://www.nobelprize.org/prizes/chemistry/1912/sabatier/lecture/
    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.
  2. APPARATUS FOR CATALYTIC REDUCTION
    Roger Adams and V. Voorhees
    Org. Synth. 1928, 8, 10
    DOI: 
    10.15227/orgsyn.008.0010
    This describes in detail how to build a reactor for catalytic hydrogenation. Nowadays these can be commercially purchased – the “Parr reactor” is very common.
  3. PALLADIUM CATALYSTS
    Ralph Mozingo
    Org. Synth. 194626, 77
    DOI: 10.15227/orgsyn.026.0077
    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.
  4. 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
    DOI: 10.1021/ja01508a029
    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.
  5. The Stereochemistry of the Hydrogenation of Cycloolefins on Supported Palladium Catalysts
    Samuel Siegel and Gerard V. Smith
    Journal of the American Chemical Society 1960 82 (23), 6087-6090
    DOI10.1021/ja01508a028
    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.
  6. 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
    DOI: 
    10.1021/ja00890a041
    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.
  7. 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
    DOI: 
    10.1021/ja01049a050
    This paper demonstrates that 1,2-dimethylcyclohexene can be reduced with a preferential syn addition from the less hindered side over Pt under H2.
  8. 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
    DOI: 
    10.1021/jo010633b
    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.

Comments

Comment section

24 thoughts on “Reagent Friday: Palladium on Carbon (Pd/C)

  1. “palladium (Pd) or platinum (Pt) are all equivalent to each other.” , “There are subtle differences in reactivity.”

    Sir , could you list out the differences?

    1. One really subtle effect is that stereocenters adjacent to a pi bond can get epimerized. Saw this one in a dihydrofuran system. Epimerization was highest in Pd(OH)2 (1:1), lower with Rh/Al2O3 (7.5:1), and even lower with PtO2 (20:1). No scrampling with HN=NH.
      Some cationic hydrogenation catalysts can be directed with pendant hydroxyl groups.

  2. Hey there! I really appreciate this website, it’s been a lifesaver since I started Organic Chemistry! I have a question about Pd/C H2 — does it reduce carbonyls as well as alkenes/alkynes?

  3. Hahaha… The matchmaker example was excellent.. That just brought the laugh.. Enjoyed reading.. Understood well.. Thanks!!!

    1. Hard to get selectivity for alkene over alkyne with H2/Pd. You’ll likely get complete reduction to the alkane, containing no double bonds. If you want selective reduction of the alkyne to the alkene, use Lindlar’s catalyst.

  4. For all of the examples given with an aromatic ring, will those reactions only if the substituent being reduced is on benzene? For example, will a regular imine also be reduced?

    1. With enough pressure, yes. Imine reduction is usually done with NaCNBH3 or LiAlH4. The one reaction where Pd-C/H2 works best next to an aromatic ring is reduction of a ketone to an alkane. Very difficult to do with a “normal” ketone but it can be done when the ketone is adjacent to an aromatic ring.

  5. By what mechanism is the H2 attacking the pi bond? And why don’t carbonyl grp don’t get reduced at normal condition even if the c=o bond the polar in nature?

    1. Well that is a very good question, and generally doesn’t get covered in an intro course. What happens is that the Pd forms a transitory bond with the sigma bond in the H-H molecule, and then, by a mechanism known as “oxidative addition” that H-H bond is broken and two new Pd-H bonds are formed. Then, Pd bonds to the alkene, and through another mechanism we don’t cover called, “migratory insertion” one of the new C-H bonds is formed, followed by a last mechanism called, “reductive elimination”. This forms the second C-H and brings the oxidation state of the Pd back to 0 so it can re-enter another catalytic cycle.

      So there’s a lot going on at the surface of the Pd that we don’t really cover here because it’s best handled in introductory inorganic chemistry classes.

      The question about reducing C=O is a longer answer – it has to do with the fact that Pd is a late metal and tends to bind better to alkenes than highly polar carbonyls.

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