Alkanes - Chemical Properties
Alkanes are a homologous series of hydrocarbons.
Alkanes can be represented by a general chemical formula.
This represents a similar functional group in all alkanes
Because they are a homologous series, alkanes show similar chemical reactions
Let's look at some chemical reactions of alkanes one by one
First, we will see the substitution reaction of alkanes.
Alkanes show substitution reaction while reacting with halogens.
For an alkane to react with a halogen, the presence of sunlight, UV rays or high temperature is necessary. In that case, one of its hydrogen is substituted by the halogen.
For example, when methane reacts with chlorine, monochloro methane is formed.
If this monochloro methane again reacts with the halogen, one more hydrogen is substituted by the halogen.
So, on repeating the process, all
of methane will be substituted. It will be converted into carbon tetrachloride.
Another important chemical reaction of alkanes is its pyrolysis.
Pyrolysis literally means the breakdown of something by heating.
Pyrolysis depends on the conditions under which it is performed
When the pyrolysis takes place under the influence of high temperature only, it is called the thermal pyrolysis.
For eg: Thermal pyrolysis of methane converts it to ethyne
Whereas when the pyrolysis takes place under the influence of a catalyst, it is called the catalytic pyrolysis.
For eg: Catalytic pyrolysis of ethane converts under influence of
converts it to ethene
This process is also called dehydrogenation of alkanes. This because hydrogen is removed from alkanes.
Now, let's see what happens when the alkane undergoes oxidation reaction.
One way to do this is the complete combustion of alkanes.
In the complete combustion of alkanes, it is oxidised in the presence of an excess of oxygen. They are converted into carbon dioxide.
For example, this is reaction of complete combustion of methane
Similarly, ethane is also converted into carbon dioxide under complete combustion
This will be the general equation of alkane undergoing complete combustion
Now, let’s discuss the incomplete combustion of alkanes.
In incomplete combustion, alkanes are oxidized in a limited amount of oxygen. They are converted into carbon monoxide and heat.
For example, this is reaction of incomplete combustion of methane
Alkanes can also be oxidized in presence of catalysts
Here, alkanes are oxidized at a high temperature in the presence of a catalyst. This process is known as controlled oxidation.
For example, methane can be oxidized in a copper tube at a high temperature. This will convert it to methanol
The catalytic oxidation of alkane with suitable catalyst gives different products.
All these oxidation processes required oxygen
Alkanes can also be oxidized using some oxidizing agents.
Generally, alkanes resist oxidation by oxidising agents
, only alkanes that have a tertiary
are oxidised. They areconverted to tertiary alcohols
But alkanes like methane and ethane can be repeatedly oxidised with the same oxidizing agent to give carboxylic acids. This process is known as slow oxidation of alkanes.
For example, methane is oxidized into methanol in the presence of
The methanol is further oxidized into methanal.
And methanal on further oxidation gives methanoic acid.
This reaction can be given as,
In substitution reactions, an alkane reacts with a halogen and one of its hydrogen is substituted by the halogen.
In the pyrolysis process, hydrogen is removed from the alkane converting it into alkene or alkynes.
Methane is burnt in the excess of oxygen to get converted into carbon dioxide and heat. The process is called complete oxidation of alkanes.
But when methane is burnt in the presence of limited oxygen, it is converted into carbon monoxide and heat. This process is known as incomplete oxidation of alkanes.
Alkanes can also be oxidized with the help of catalysts at high temp. This process is known as catalytic oxidation.
Only alkanes with tertiary
are also oxidized with the help of an oxidizing agent. This is called slow oxidation of alkanes.
Methane and Ethane can undergo called slow oxidation