Preparation of alkanes

Hydrocarbons are organic compounds that contain hydrogen and carbon atoms only.

Hydrocarbons are classified as:

Alkane comes under the class of saturated hydrocarbons.

These alkanes are predominantly found in nature.

To get pure alkanes we need to prepare them in the laboratories.

These alkanes can be prepared by a number of reactions.

Two major ways to produce alkanes are: $\text{i) Kolbe’s electrolysis }$ $\text{ii) Hydrogenation reaction}$

Let’s discuss the Kolbe’s electrolysis first.

In this, sodium salt of carboxylic acid $(RCOONa)$is electrolysed to produce an alkane $R-R$

In order to form ethane $(C{H}_3-C{H}_3)$ we will use sodium acetate $(C{H}_{3}COONa)$

The setup for the reaction is two rods dipped in a solution of aqueous sodium acetate. The rods are connected to a battery

On electrolysis, ethane is produced at the anode.

To understand this better, let us look at the mechanism of Kolbe's electrolysis.

Kolbe’s reaction is a combination of the ionic and free radical mechanism.

The Sodium salt of carboxylic acid first ionises to carboxylate ion.

Carboxylate ion formed moves towards the anode.

The electrons rearrange in the way shown above. This leads to a loss of $C{O}_2$ and formation of methyl free radical

The two methyl free radicals generated then combine to form ethane.

At the cathode, the water molecule gets ionised. $H^{+}$ is reduced to form hydrogen free radical.

The two hydrogen radicals then combine to form $H_2$ gas.

Another way alkanes are prepared is by the hydrogenation reaction.

Hydrogenation reaction involves addition of hydrogen to unsaturated hydrocarbons.

The Addition of hydrogen to alkene or alkyne produces an alkane.

Let's take the example of addition of hydrogen to ethene.

Here, the double bond between the ethene molecule breaks and hydrogen is added to each carbon atom.

The reaction is carried out in the presence of a catalyst called Raney Nickel

Raney nickel is an alloy of aluminium and nickel. It provides the larger surface area for the adsorption of hydrogen gas.

Let’s discuss the mechanism for the catalytic hydrogenation of ethene.

First, hydrogen atoms get attached to the metal catalyst surface. And then the alkene attaches itself to the catalyst surface.

Next, one hydrogen from the catalyst surface is transferred to the alkene.

And then the second hydrogen is transferred to the alkene leading to the formation of a saturated alkane.

REVISION

Alkanes are the simplest saturated hydrocarbons with general formula $C_n{H}_{2n+2}$.

Alkanes can be produced by electrolysis of sodium salt of carboxylic acids. This is called Kolbe’s electrolysis reaction

Hydrogenation of alkene and alkyne give alkanes.

The End