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What is Catalytic Hydrogenation? - Mechanism & Reaction

Instructor: David Jones

David has taught General and Organic Chemistry and has a master's degree in Chemistry.

In this lesson you will learn all about catalytic hydrogenation. In addition to its definition, you will learn what is needed to perform catalytic hydrogenation and its reaction mechanism.

Tubing Down the River

Have you ever been tubing down a river? You put your bathing suit on, sunscreen if needed, and get in your inner tube as the current slowly takes you downstream.

Now imagine that your family wants to go tubing but does not want to be separated. Your parents may decide to hold hands in order to stay together, leaving one hand and two feet for all of the little children to grab onto. This is like two carbon atoms holding on to each other, leaving three spots each for all of the little hydrogen atoms to attach.

Carbon are the parents and hydrogen are the children
Ethane

However, sometimes the parents may want to hold both hands, leaving only their legs (two each) for the kiddos to hold onto, similar to an alkene.

Alkene
Ethylene

Or, the parents may even hold both hands and hook one foot together, leaving only one foot each for their kids. This is similar to an alkyne, or triple bond. Each time you add a bond between two carbons, you leave fewer spots for other things, such as little hydrogen kiddos to grab onto.

Alkyne
Acetylene

Catalytic Hydrogenation

Hydrogenation means to treat with hydrogen. In other words, it means adding hydrogen (or adding more kids) so that the carbon atoms (or parents) have fewer bonds between themselves. A compound that has been fully hydrogenated looks a lot like the parents who are only holding one hand, leaving two feet and one hand each for the kids to hold onto.

So if hydrogenation means adding hydrogen to a double or triple bond, catalytic hydrogenation means using a catalyst to add hydrogen. In other words, in order to add more kids to the tubing family, you need an aunt or uncle on the shore guiding the kids into the water!

Mechanism

During catalytic hydrogenation, the metal catalyst (typically nickel, paladium or platinum), a solvent, and the compound to be hydrogenated are all added to a flask. This flask is then put under pressure of hydrogen gas. The hydrogen gas provides an excess of H2 molecules to the reaction.

The first step in the reaction mechanism is that the hydrogen gas, or H2, becomes bonded to the metal catalyst because of the pressurized gas. This is done by a hydrogen molecule, with its two hydrogen atoms, first approaching the surface of the metal. Each atom of the hydrogen molecule then trades its bond with the other hydrogen atom for a bond to the metal surface, leaving two separated hydrogen atoms now attached to the surface of the metal. Since an abundance of hydrogen is used, the metal catalyst is soon covered with a layer of hydrogen atoms.

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