Return to Contents Chapter 1: Structure Determines Properties
Constitutional Isomers

Compounds that have the same molecular formula but different chemical structures are called isomers. Remember isomerism is a property between a pair (or more) of molecules, i.e. a molecule is an isomer of another molecule. A similar relationship is that of brother or sister... you can only be a brother or sister to someone else.

Depending on the nature of the difference between the structures, it is possible to classify isomers into various sub-types. The following tree diagram should help you recognise the differences based on a simple YES / NO question.  If you "click" on the named boxes there is a link to a definition and an example.

starting an isomer tree


Isomers are compounds with the same molecular formulae but that are structurally different in some way. It is important to be able to recognise isomers because they can have different chemical, physical properties and biological properties.


Constitutional (or structural) isomers differ in the order in which the atoms are connected together so they contain different functional groups and / or bonding patterns (e.g. branching)
  • example: constitutional isomers of C3H8O are propan-1-ol, propan-2-ol and ethyl methyl ether


Stereoisomers contain the same functional groups and branching patterns, they differ only in the arrangement of atoms in space. 


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Dr. Ian Hunt, Department of Chemistry, University of Calgary