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Constitutional Isomers

Article ID: 121
Last updated: 17 Jun, 2013
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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 recognize the differences based on a simple YES / NO question.



Isomers are compounds with the same molecular formula but that are structurally different in some way. It is important to be able to recognize 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: isomers of C3H8O are 1-propanol, 2-propanol and ethyl methyl ether


Stereoisomers contain the same functional groups and branching patterns, they differ only in the arrangement of atoms in 3D space. 


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Organic Chemistry -> Alkanes: Structure and Conformational Analysis -> Isomeric Alkanes
Organic Chemistry -> Introduction to Organic Compounds and Reactions -> Drawing Organic Compounds

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Department of Chemistry & Chemical Biology