Catalytic Hydrogenation of Alkenes: Mechanism & Explanation

Instructor: Hemnath (Vikash) Seeboo

Taught Science (mainly Chemistry, Physics and Math) at high school level and has a Master's Degree in Education.

Alkenes undergo addition reactions with hydrogen under certain conditions. This lesson will walk you through the mechanism behind hydrogenation of alkenes.

Margarine vs Butter

Jenny's made baked potatoes, but realizes that she's out of butter to put on them! Her father told her to use the margarine instead. Later the same evening, Jenny made an internet search on margarine and concluded that it's basically made from unsaturated vegetable oils such as olive oil and rapeseed oil.

Let's go into the details of Jenny's conclusion.

Addition Reactions of Alkenes

Before proceeding any further, it is important to recall that the most important reaction of alkenes is the addition reaction. While alkenes are unsaturated compounds containing at least one double bond (made up of a sigma and a pi bond), an addition reaction is one where the alkene reacts with another molecule to produce a single product.

Addition reaction of an alkene

As you might have also noticed, an addition reaction converts an unsaturated reactant into a saturated product, where single bonds are formed when the double bond is broken.

Hydrogenation of Alkenes

One of the most significant industrial addition reaction involves the addition of hydrogen to an alkene. The reaction is also known as the hydrogenation of alkene.

The reaction occurs in the presence of a finely divided metal catalyst such as nickel (Ni), palladium (Pd), platinum (Pt) or rhodium (Rh) at a temperature of about 150°C. This is what precisely occurs when unsaturated vegetable oils are converted to saturated margarine.

A few different examples will help you understand.

Hydrogenation of ethene to form ethane

Hydrogenation of 2-methyl but-1-ene to form 2-methyl butane

Hydrogenation of 3,5-Heptadiene to form Heptane

Mechanism Behind Hydrogenation of Alkenes

To explain the mechanism behind hydrogenation of alkene, we are going to use the reaction of ethene with hydrogen in the presence of nickel catalyst at 150°C. The mechanism can be described in four different steps.

Step 1: Ethene molecules adsorbing onto the nickel surface

In the first instance, it would be fair to recall that adsorption is not the same as absorption. Adsorption is the adhesion or sticking together of molecules onto the surface of a solid or liquid.

Initially the ethene molecules are adsorbed on the surface of nickel catalyst.


Once an ethene molecule is adsorbed, there is bond breaking and bond formation. The pi bond of the double bond breaks and new bonds between carbon and the surface of the catalyst are formed.


Step 2: Hydrogen molecules adsorbing onto the nickel surface

In a similar manner, hydrogen molecules are adsorbed onto the same nickel catalyst. The hydrogen-hydrogen bond breaks and new bonds between hydrogen and the catalyst surface are formed


Step3: Hydrogen gets attached to the ethene molecule

The bonds between hydrogen and the catalyst surface break and hydrogen atoms are formed.


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