• A form of stereoisomerism (geometric isomers) which describes the relative orientation or geometry of functional groups within a molecule.

• It can only be used for alkenes

• In naming the molecule, one must consider the position of the prioritized element or compound. Cis is applicable if both of the prioritized element/ compound are at the same side while Trans can be used if the two prioritized element/ compound are at the opposite side.

• Cis-trans have the same name, same count of hydrogen and carbon atoms.

• cis- if the two alkyl groups, R-, are on the same side of the C=C

• trans- if the two alkyl groups, R-, are on opposite sides of the C=C.

• these terms are inserted into the name as prefixes.

 

EXAMPLE:

 

 

 

 

 

 

 

 

 

 

 

 

• Prioritize an element/ Compound

As seen in the picture above, CH3 or the methyl groups is prioritized since is Carbon has a higher atomic number compared to hydrogen.

 

• Finding the location of the prioritized element and Naming the Molecule

The prioritized compound in the picture is methyl. The methyl groups are in the same location or side of the double bond, since the structure has 4 chain, thus the correct name for the molecule should be (Cis)-2-butene.

 

 

EXAMPLE: (TRANS)

 

 

 

 

 

 

 

 

 

 

 

 

 

• Prioritize an element

In the picture, chlorine group and hydrogen group is branched in the C=C. in this case, Chlorine is prioritized since it has a higher atomic number compared to hydrogen. Therefore, the chlorine group is prioritized.

 

 

• Finding the location and naming the molecule

The location of the prioritized chlorine group. Since they are in the opposite location, therefore, trans isomer will be used. Since there are 2 chlorine, the Greek prefix di- will be used. The location of the chlorine groups is at 1 and 4, and the structure has 4 chain which corresponds to butene, it will be named as (trans)-1,4-dichloro butene.

 

* CAHN-INGOLD PRELOG PRIORITY RULE:

-it can be used in Cis-Trans Isomerism by using the following steps:

 

1. Prioritize according to atomic number (higher atomic number, therefore higher priority)

2. If the same atoms are bonded to C=C chain, then go further from the 2 bounded-bond until differences is found.

 

 

 

Sources:

http://en.wikipedia.org/wiki/Cis–trans_isomerism

http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

https://www.khanacademy.org/science/organic-chemistry/alkenes-alkynes/naming-alkenes/v/cis-trans-and-e-z-naming-scheme-for-alkenes

 

 

 

 

written by: Sydney Santiago