Introduction to constitutional (structural) isomers

Constitutional (structural) isomers are compounds that have the same molecular formula but a different atom-to-atom bonding sequence.

The constitutional isomers below each have a different carbon skeleton (arrangement of carbon atoms). Therefore, they have different physical properties and can be separated from each other.
 

Compounds that have the same carbon skeleton but have a double bond between different carbon atoms are also constitutional (structural) isomers. Two such isomers are shown below.