What are some common mistakes students make with E-Z isomerism?

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Nov 9, 2015


There are many mistakes students trip on that will be explained below.


To make it easier for us, we can call E-Z isomerism as Cis -Trans isomerism. One particular mistake that students often do is incorrectly distinguishing the difference between Cis and Trans isomers.

For instance, a Trans isomer is one containing a #C=C# double bond with two different functional groups. The two functional groups have to be on the opposite planes from each other. To remember this we simply think of the Trans-Atlantic ocean as being across the ocean. See the image I made on paint below:
enter image source here

Now, the Cis isomer is the exact same as the Trans isomer, however, now the functional groups are opposite each other i.e. face to face. See the image I made on paint below:
enter image source here

Another tripper is that students forget to know when a molecule shows Cis -Trans isomerism. A Cis -Trans isomer needs to have two things in its molecule:

  1. A #C=C# double bond = This is what gives a molecule the definition of a Cis -Trans isomer. The double bond restricts rotation of one side of the molecule, so you simply can't twist it to yield both isomers.

  2. Different functional groups on each #C# of the #C=C# double bond = The two carbon atoms of the double bond must be bonded to two different functional groups i.e. you cannot have a carbon bonded to two hydrogen atoms, since the two hydrogen atoms are identical to each other.

Easy Peasy. I hope I answered your question correctly.

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