Alkanes - Preparation
Alkanes are very important compounds with many uses
But Alkanes are not available in pure form.
In nature, alkanes are found in the mixture form with other chemical compounds.
To get the alkanes in its pure form, we can do two things.
Method 1: We can isolate alkanes from its mixtures.
Method 2: We can prepare these alkanes in our labs involving some chemical processes.
We will look at both of these methods
Let’s first discuss how the alkanes are extracted from natural sources
Natural gas and the petroleum are the main sources of the natural occurrence of these compounds.
Natural gas mainly consists methane. We also find some heavier alkanes in these gases but in a small amount.
Petroleum contains mainly heavier alkanes along with some other hydrocarbon compounds.
Using the distillation, we isolate liquid and solid alkanes from these mixture.
Now, let’s understand how the alkanes are prepared in the chemical labs.
In labs we can prepare alkanes by three methods:
Decarboxylation using soda lime.
Reduction of alkyl halide
Let's see the first method i.e Decarboxylation using soda lime
In this reaction, our reactants will be sodium ethanoate and soda lime. Using these will give us methane
Soda lime is the mixture of sodium hydroxide and calcium oxide.
To prepare methane, we use the above apparatus in the lab
At first, the mixture of sodium acetate and soda lime is taken into a hard glass tube.
Then, we heat this mixture with the help of a burner.
This is heated upto
and it gives away methane gas.
The gas passes through the water and collected in the upper side of the inverted gas jar.
This method can be used to prepare all the alkanes. Let's see how ethane will be prepared
To prepare ethane, we take the mixture of sodium propionate and soda lime.
The process can be chemically expressed as,
This time, ethane gas is collected in the inverted gas jar.
Now, if we look at both these reaction, we find that carbon dioxide is removed from the carboxylic acids.
Due to the removal of the carbon dioxide, these reactions are also called decarboxylation reaction.
Now, let's see the second method i.e reduction of alkyl halides
In this method, alkyl halides are reduced to alkanes with the help of nascent hydrogen.
Here, the nascent hydrogen is obtained from Zinc and Hydrochloric acid solution.
So Zinc and
behaves as the reducing agent for the alkyl halides
We obtain methane from methyl iodide as,
Here, we can note that the alkane obtained from this reaction is same as the alkyl group in the alkyl halides.
Finally we will see Wurtz reaction
In this reaction, alkyl halides react with sodium in the presence of dry ether and give us higher alkanes.
For example, methyl iodide reacts with sodium in the presence of dry ether to give us ethane.
This reaction is known as Wurtz reaction.
Wurtz reaction is not preferred to create odd numbered alkanes. This is because unwanted products are created
For eg: In wurtz reaction of bromoethane and bromomethane, Along with desired propane, undesired butane and ethane are produced
The desired product is propane is produced as shown above
The undesired products Ethane and butane are are produced like above
Alkanes are naturally found in natural gas and petroleum.
We can prepare methane gas in the chemical labs by heating the mixture of sodium acetate and soda lime.
The lab preparation of methane gas can be shown as,
The reaction in the lab preparation of alkanes are also called decarboxylation due to the removal of carbon dioxide.
Alkyl halides can also be reduced to alkanes with the help of hydrogen using zinc and hydrochloric acid.
The reaction of alkyl halides in such cases can be given as,
We can also obtain higher alkanes using alkyl halides with sodium in the presence of dry ether. The reaction is known as Wurtz reaction.