Isomerisation and Other Reaction of Alkanes.
Alkanes are hydrocarbons that contain only single bonds
Due to this, the majority of reactions that alkanes show are substitution reactions
However, alkanes can also show some special reactions
Some reactions shown by alkanes are:
iii) Reaction with steam
iv) Pyrolysis or cracking
Let’s discuss these reactions one by one
First, let’s look at isomerization
Isomerization is the process by which one isomer of alkane is changed into another isomer
Isomers are compounds having the same molecular formula but the different structural formula
In isomerization, N-Alkanes are heated in the presence of anhydrous
. This isomerizes them to branched-chain alkanes.
For example: Isomerisation of n-butane to iso-butane occurs as shown above
forms two isomers on isomerization as shown above
Next, we will see the aromatization reaction of alkanes
In aromatization reaction, a non-aromatic hydrocarbon is converted to an aromatic hydrocarbon.
N-Alkanes having six or more carbon atoms can lose hydrogen and get cyclised to benzene and its homologues.
This requires heating them at high temperature in the presence of a suitable catalyst
The Catalyst used in these reactions are oxides of molybdenum or chromium supported over alumina.
For eg: n-hexane can be converted to benzene at a temperature of
and at a pressure of
Similarly N-heptane on heating in presence of
first forms methylcyclohexane.
This methylcyclohexane on heating with metal like platinum forms toluene.
Alkanes can also react with Steam.
The Reaction of methane with steam occurs at a high temperature in the presence of nickel catalyst.
This method is used for the preparation of
Alkane also undergo cracking
Decomposition of higher alkanes into lower alkanes and alkenes by the application of heat is called cracking.
When this is achieved by using high temperature and pressure without a catalyst, it is termed as thermal cracking.
For example, consider the thermal cracking of hexane into other hydrocarbons.
Here, we can see there are possibilities of the formation of various hydrocarbons. This makes thermal cracking hard to control.
To overcome this problem, we use catalytic cracking. In such reactions, higher hydrocarbons are broken into lower ones using catalysts.
Catalytic cracking is carried out at a relatively lower temperature and pressure than thermal cracking for similar hydrocarbons.
For example, the catalytic cracking of dodecane gives a mixture of heptane and pentene.
Isomerization is the process by which one isomer of an alkane is transformed into another isomer
Aromatization is the process in which benzene and its homologous are formed from alkanes having six or more carbon atoms.
Methane reacts with steam to produce
and hydrogen gas
Decomposition of higher alkanes to lower alkanes, alkenes etc. at higher temperature is called pyrolysis or cracking.