Alkanes: Isomerism

Print Friendly, PDF & Email

Homologous series

Alkanes have a general formula of CnH2n+2. The simplest alkane is CH4 where n = 1. The next alkanes are C2H6 (n = 2), C3H8 (n = 3), and so on so forth. Notice, there is a steady difference of number of carbon and hydrogen atoms from lower alkanes to higher alkanes, the constant difference between successive members being CH2. A series of compounds in which each member differs from the next member by a constant amount is called a homologous series, and the members of the series are called homologs.


Alkanes with more than 3 carbons can show constitutional isomerism. They can be either linear or branched structures. This is categorized as chain isomerism. Butane is the smallest alkane to show such isomerism with 2 isomers. Pentane can have 3 isomers.

Alkane isomer
Figure 1. Isomers of butane and pentane

Similarly, one can calculate isomers for higher alkanes. However, as the number of carbons increase, it becomes very hard to calculate the number of isomers. This particular problem has been a mathematical challenge for almost a century. A detailed description of this problem can be found here.

The Goodman group from the University of Cambridge has a calculator, which you can use to find the number of isomers of alkanes.


[stextbox id=”info” caption=”Sample problem 1″]Find out the number of isomers of C6H14 from the above calculator and then try to draw the structures of all of them.

Try it yourself.