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Isomerism Index

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Creative Chemistry Molecular Models

Geometrical Isomerism

What is it?

Geometrical isomerism is an example of stereo-isomerism. This occurs when substances have the same molecular formula, but a different arrangment of their atoms in space. There are three ways that this can happen:

  • where there is a C=C bond in the molecule;
  • where a molecule has rings; or
  • where there is a >C=N bond.

In AS and A2 Chemistry, we only need to know about geometrical isomerism caused by a C=C bond in the molecule.

If you are studying Biology, you will meet geometrical isomerism caused by rings when you look at sugars such as glucose, fructose, mannose and galactose. Go to the Biofiles pages to see these.

What is here?

I have put models of the geometrical isomers of but-1-ene and but-2-ene here. But-1-ene does not form geometrical isomers, even though it has a C=C bond, because one of the double-bonded carbon atoms has two identical groups on it (hydrogen atoms in this case). But-2-ene does form geometrical isomers because each double-bonded carbon atom has two different groups on it. You should be prepared to spot geometrical isomers for simple organic compounds like these for your examinations, and you also need to be able to name the them.



Where like groups are on the same side of the double bond, we call it a cis isomer; where they are on opposite sides we call it a trans isomer.





Although but-1-ene contains a C=C bond,
it does not form geometrical isomers.

Take care - look for different groups
on the double-bonded carbon atoms!


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The models are ball-and-stick, but you can right click to get a menu of options, such as animations and changing to space-filling models. You can also move the models using the left mouse button.

Molecular modelling applet courtesy of ChemAxon Ltd


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