Chain isomerism: When two or more compounds have the same molecular formula but different parent chain or different carbon skeletons, it is referred to as chain isomerism and such isomers are known as chain isomers.
e.g.
i. Butane (C4H10) exists in two isomeric forms:
Here, n-butane contains longest chain of four carbon atoms whereas isobutane contains longest chain of three carbon atoms. Such isomers having different carbon skeletons are called as chain isomers.
[Note: Methylpropane has no other branched isomers, hence locant (2) can be dropped.]
ii. Pentene (C5H12) exists in three isomeric forms:
[Note: The numbers of chain isomers increase with the increase in the number of carbon atoms in the molecule.]
