The Difference Between Structural & Geometric Isomers

Updated April 17, 2017

Isomers are identified as two or more chemical compounds that have identical molecular compositions but have different structural arrangements. The two basic types of isomers are structural and stereoisomers. Geometric isomers are a subcategory of stereoisomers. It may sound complex at first, but the differences between structural and geometric isomers aren't too difficult to grasp.

Naming Molecules

Molecular composition is the most basic aspect of any chemical compound. Scientists use molecular composition to identify chemicals. For example, the molecular composition of butane is C4H10, meaning there are four carbon atoms and 10 hydrogen atoms in any single molecule of butane. The difference between isomers becomes apparent in the structural bonds and spatial organisation of molecules. Such seemingly small differences have vast implications in the molecular world.

Structural Isomers

Structural isomers are molecules with identical atomic composition, but differ in how the atoms are arranged and bound together. The atoms in a butane molecule, for example, are structured into a straight chain (CH3-CH2-CH2-CH3); a isobutane molecule, an isomer of butane, has a central CH atom to which are bonded three CH3 atoms forming a "T" shape. Structural isomers form straight bonds, unlike the diagonal bounds typical of geometric isomers.

Geometric Isomers

Geometric isomers, or "cis-trans" isomers, are molecules that share identical atomic compositions and molecular bonds but have different spatial organisation. Additionally, geometric isomers are incapable of rotating and are restricted to their form. The geometric isomers cis-2-butene and trans-2-butene, for instance, have the identical molecular composition of C4H8. Both isomers feature two methyl groups and a central carbon-carbon double bond. They differ in spatial organisation because cis-2-butane molecule features the two methyl groups on the left side of the central carbon-carbon double bond, while the trans-2-butane molecule features the methyl groups on the opposite side. The central carbon-carbon double bond prevents the molecules from rotating, thus the two methyl groups stand no chance of switching sides.

Behavioural Properties

Such small differences are reflected in each isomer's unique physical and chemical properties. The differing bonds that form structural isomers have the capacity to alter the chemical substance into a completely different substance with its own chemical properties. Spatial organisation retains most chemical properties in geometric isomers, but they have difference physical properties. These differences are dependent on the electrical charge, polarity and cis-trans group of the isomer. These can be major differences or small, for example, trans-2-butene's freezing point is minus 105 degrees Celsius, while cis-2-butene's freezing point is minus 139 degrees Celsius.

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About the Author

Based in Victoria, British Columbia, Sebastian Malysa began his writing career in 2010. His work focuses on the general arts and appears on Answerbag and eHow. He has won a number of academic awards, most notably the CTV Award for best proposed documentary film. He holds a Master of Arts in contemporary disability theater from the University of Victoria.