Constitutional 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. This means that if two constitutional isomers are present in a mixture they can be separated from one another.
Strategy for identifying compounds that are constitutional isomers of one another:
Find compounds with the same molecular formula.
For alkanes this means that it is simply necessary to count the carbons!
In case you are wondering why the term constitutional isomer is emphasised in preference to structural isomer:
The term structural isomer is ambiguous because organic chemists use the term structure in a general way, not only to describe the atom-to-atom bonding sequence. As you progress in chemistry you will learn about another type of isomerism where isomers having the same molecular formula AND the same atom-to-atom bonding sequence may have different structures due to differing arrangements of the atoms in space.
The constitutional isomers below each have a different carbon skeleton (arrangement of carbon atoms). Therefore, they have different physical properties. This means that if two constitutional isomers are present in a mixture they can be separated from one another.
 |  |  |
| C6H14 | C6H14 | C6H14 |
| boiling point 63 °C | boiling point 50 °C | boiling point 58 °C |
Strategy for identifying compounds that are constitutional isomers of one another:
Find compounds with the same molecular formula.
For alkanes this means that it is simply necessary to count the carbons!
In case you are wondering why the term constitutional isomer is emphasised in preference to structural isomer:
The term structural isomer is ambiguous because organic chemists use the term structure in a general way, not only to describe the atom-to-atom bonding sequence. As you progress in chemistry you will learn about another type of isomerism where isomers having the same molecular formula AND the same atom-to-atom bonding sequence may have different structures due to differing arrangements of the atoms in space.
The term constitutional is unambiguous because it implies that the isomers have the same constitution (molecular formula).