Hydrogenation demonstration

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>> Wednesday, November 18, 2009

Before going to an experimental section, let’s have a brief introduction about hydrogenation. Hydrogenation,  addition of hydrogen atoms to molecules, is used to reduce or saturate various organic compounds. Hydrogen gas is the common source of hydrogen and palladium on activated charcoal (Pd/C), platinum (Adam’s catalyst) and raney nickel are the most common catalyst. Reduction of alkene to saturated alkane is the simplest hydrogenation. Hydrogenation can also be used to reduce alkyne to alkene and finally to alkane. Use of Pd/C results complete reduction of alkyne to alkane whereas less active catalyst like Lindlar catalyst gives alkene.
#Lindlar catalyst: finely divided palladium, precipitated onto calcium carbonate and then deactivated by lead acetate and quinoline.
General mechanism of hydrogenation (of alkene) starts with the adsorption of H2 on the catalyst surface. After that alkene forms the complex with catalyst. Insertion of hydrogen into carbon-carbon double bond occurs and finally saturated alkane releases from the catalyst.
I think I shall stop on introduction part and rather come to the main aim of this post. Actually, I want to show how to setup the hydrogenation reaction using hydrogen balloon and Pd/C catalyst.At first, reagents, solvent and catalyst are kept in the RBF. Before adding catalyst to the solvent be sure to remove the air by flushing with inert gas like nitrogen. Thereafter, one end of the connector is joined to the RBF and another end (opposite to RBF) to the hydrogen balloon. Third outlet is for evacuating the RBF by connecting to the vacuum. Evacuation is repeated for few times and finally the hydrogen gas is passed by opening the upper valve. Evacuation should be done carefully. During my early days, I had bad experience of evacuating the reaction mixture and loosing the compound. The reaction was kept until the completion of starting material (TLC checking). On completion of reaction, Pd/C was removed by filtration through celite, residue washed with appropriate solvent and the filtrate was concentrated to get the desired compound. If necessary, further purification is performed.



I hope it’s helpful for you and easy to understand. Check out the video for more understanding. I was thinking to show the hydrogenation using the hydrogenation apparatus too but unable to do so. I'll try to post it very soon.



4 comments:

  1. Yes, even in industry (to carry out small rexns), I have used this method. A small precaution one need to follow while filtering is, to use nitrogen atmosphere (blanketing) otherwise even a small amt of moisture will lead to fire (un reacted/ excess of Pd/C)....

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  2. @syn-chemist, good to hear from you. Ya this is convenient for small scale reactions.

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