Catalytic Hydrogenation of Alkenes | Reaction of Alkenes with Hydrogen

Alkenes readily react with hydrogen gas in the presence of catalyst and give saturated alkanes as the product. Otherwise, we can say alkenes are reduced to alkanes in the hydrogenation.

In this tutorial, we will learn followings.

Products of alkene hydrogenation
Types of catalyst and their properties in the reaction
Required temperature
Examples of hydrogenation of alkenes
Hydrogenation when more than two double bonds

This reaction is defined as hydrogenation of alkene. Two hydrogen atoms are added across the double bond of alkene. Also carbon atom is reduced due to addition of hydrogen atoms (formation of new C-H bonds) because electronegativity of carbon is higher than hydrogen.

Catalyst used in the hydrogenation

Platinum (Pt) or Palladium (Pd) or nickel or Raney nickel are used as catalyst in hydrogenation of alkene.

The main task of catalyst is activate the molecules of hydrogen.

Platinum as the catalyst in the hydrogenation of alkene

Platinum is the most efficient catalyst for alkene hydrogenation. But it is a so expensive element. So using platinum in the laboratory scale is not practical.

Nickel as the catalyst

Nickel is not so much expensive like platinum. Thus, Nickel is used as the catalyst in most times. But Nickel can be undergone catalyst poisoning (partial or total deactivation of the catalyst). That will disturb to the continuity of the reaction.

Temperature of the reaction

With platinum or palladium or Raney nickel catalysts, reaction occurs at room temperature. But, with ordinary nickel, temperature has to be increased upto 523 - 573 K.

Examples of alkene hydrogenation

Ethene and hydrogen gas reaction

Ethene and hydrogen gas reaction in the presence of nickel or platinum or palladium catalyst will give ethane as the product. Room temperature is enough to occur the reaction. Ethane is a saturated alkane which has two carbon atoms.

CH₂=CH₂ + H₂ → CH₃CH₃

Propene and hydrogen gas

Propane, a saturated alkane with three carbon atoms is given as the product when propene reacts with hydrogen gas in the presence of one of the above catalyst. One mol of hydrogen gas is added to the one mol of propene.

CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃

Hydrogenation of alkene when there are more than two double bonds between carbon atoms

There is no difference in the reaction when there are more than two double bonds between carbon atoms. The important thing is you have to feed more hydrogen gas to react with double bonds.

Hydrogenation of 1,4-dihexene

In 1,4-dihexene, there are two double bonds between carbon atoms. So to react with 1 mol of 1,4-dihexene, 2 mol of hydrogen gas are required.

Uses of the catalytic alkene hydrogenation

In the manufacturing of vanaspati ghee from vegetable oil.

Questions asked by students. Ask your question and find the answer free.

Does always require 1 mol of hydrogen gas for hydrogenation of 1 mol of alkene

Alkene can contain one or several double bonds. If there is only single double bond in an alkene, at least 1 mol of hydrogen gas is required for 1 mol of alkene. Otherwise, you need more hydrogen gas for hydrogenation of alkenes which have two or more double bonds.

How much hydrogen gas amount is added to the alkene?

If alkene molecule have only one double bond, one hydrogen gas molecule is added across the double bond.

What are the other ways of preparing alkanes?

There are more ways to prepare alkanes like hydrogenation of alkenes.

How, alkene hydrogenation and alkyne hydrogenation is different?

As alkene hydrogenation, alkynes can be hydrogenated by hydrogen gas in the presence of platinum or palladium or nickel catalysts. But for alkyne hydrogenation, required amount of hydrogen gas is higher than alkene hydrogenation.

In the hydrogenation, hydrogen atoms are added to alkenes. Then, what will happen in alkene hydration?

Hydrogenation and hydration is two different things. In hydration of alkenes, a water molecule is added across the double bond.

In alkene hydration, one carbon atom in the double bond is oxidized and other one is reduced. But, in the hydrogenation of alkene, both carbon atoms are reduced.

hydrogenation of alkenes examples

Ethene reacts with hydrogen gas in the presence of Pt catalyst will give ethane. (Hydrogenation of ethene gives ethane).
2-butene reacts with hydrogen gas in the presence of Pt or Pd catalyst will give butane. (Hydrogenation of 2-butene gives butane).

Can I produe methane as a product of hydrogenation of alkene?

You cannot. Methane is an alkane with a single carbon atom. For alkene, at least there should be two carbon atoms to make a double bond (simplest alkene is ethene). In alkene hydrogenation, number of carbon atoms is not changed. Therefore, by hydrogenation of alkene, the simplest alkane can be produced is ethane which has ttwo carbon atoms. So, methane is not possible to produce from hydrogenation of alkenes.

What is the difference between hydrogenation of alkenes and hydration of alkenes?

hydrogenation is the addition of hydrogen atoms to the alkene. Hydration is addition of water molecule to the alkene.

By hydrogenation of alkene, alkane is given as the product. But hydration of alkene gives alcohols as products.