Homologous series

Alkanes have a general formula of C_nH_2n+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.

Isomerism

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.

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 C₆H_14 from the above calculator and then try to draw the structures of all of them.

Try it yourself.

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