What are cis and trans geometric isomers?

1 Answer
MeneerNask
Feb 9, 2016

These isomers occur around double #C=C# bonds.

Explanation:

While everything around a single #C-C# bond is free to rotate, the double bond is not flexible. Now the attachments on the double-bonded #C#'s can be on the same side (cis-):
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or on the opposite side of each other (trans-) :
enter image source here
Since there is no rotation possible around the double bond (in the middle) these two can not be converted into one another.

Extra
Instead of using the prefixes cis- and trans- the use of the prefixes Z- and E- are in common use.
The lower picture would be called trans-but-2-ene or E-but-2-ene
(and since the cis/trans can only happen if the double bond is at place 2, the 2 is often omitted, so trans-butene or E-butene)

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