Substitution Reactions of Alkanes

Alkanes are the least chemically reactive class of organic compounds.

This is due to the presence of a strong and bonds in them.

However, one of the types of reaction that all alkanes show are free radical substitution reactions

A reaction in which hydrogen atoms of alkanes are substituted is known as substitution reaction.

Substitution reactions shown by alkanes are halogenation, nitration and sulphonation.

First, let us start with the nitration of alkanes.

Nitration of alkane occurs in the vapour phase at high temperature. Hence, it is also called vapour phase nitration.

Nitration of alkane occurs in the presence of .The product formed is called nitroalkane.

In nitration one hydrogen atom of alkane gets substituted by the group.

Let’s see the mechanism of nitration of an alkane.

Nitration of alkane follows a free radical mechanism.

Methane can undergo nitration under right conditions but the yield is low.

However, ethane can undergo nitration very easily.

Nitration of some higher branched chain alkanes can occur as shown above.

Formation of a major and minor product will depend upon the type of hydrogen which is substituted.

Substitution of tertiary hydrogen will form a major product while secondary and primary hydrogen will give the minor product.

Sometimes nitration at high temperature leads to cracking

At high-temperature bond breaks and cracking occurs. This leads to the formation of different types of radicals.

These alkyl radicals are then combined with to form various undesired nitroalkanes.

Cracking can occur in many ways. One more way is shown above

These will be the nitroalkanes produced in this case. These are also undesired products

Hence, this method is not used to prepare nitroalkanes as this method gives many undesired products.

Another type of substitution reaction of alkanes is sulphonation.

Sulphonation of alkanes occurs at a high temperature in the presence of oleum i.e

Lower alkanes do not undergo sulphonation. However, an alkane with six or more carbon shows sulphonation easily.

The reaction involves a substitution of group to an alkane.

The product formed is called alkyl sulphonic acid i.e .

Sulphonation of alkane occurs via a free radical mechanism.

The oleum used in the reaction will provide sulphuric acid in the reaction.

At high temperature, the bond between and breaks and free radicals are generated.

These free radicals will attack an alkane molecule and carry out a substitution reaction.

The rate of reaction will depend on the stability of the intermediate radical.

Stability of intermediate radical depends upon the type of hydrogen that is substituted.

Tertiary hydrogen will be substituted faster by a sulphonic group than secondary or primary hydrogen.

REVISION

Substitution reaction of alkanes involves the removal of a hydrogen atom with another atom.

Substitution reactions shown by alkanes are halogenation, nitration and sulphonation.

Halogenation involves replacement of hydrogen by a halogen group.

Addition of group to an alkane is known as nitration.

Addition of group to an alkane is known as sulphonation.

The End