Cis Trans Isomers

Core Concepts

In this tutorial, you will learn to differentiate between two types of stereoisomers: cis and trans. You will learn how to classify the different isomers, and to understand the difference in properties for cis trans isomers.

Topics Covered in Other Articles

What are Cis and Trans Isomers?

Geometric isomerism is a type of stereoisomerism where rotation around a bond is restricted, such as in the case of a double bond. Cis and trans isomers are types of geometric isomers, where the functional group is placed differently with regards to the double bond.

In general, a cis isomer has molecules on one side of the double bond. A trans isomer has molecules on the other side of the double bond. Spot the differences in the the example below:

The cis molecule is on the left: the methyl groups are both on the same side side of the double bond.
The trans molecule is on the right: the two methyl groups are on opposite sides of the double bond

Identifying Cis vs Trans Isomers

In order to identify the cis and trans isomers it is always best to spot the functional groups on the bond. Remember that the bonds will also have hydrogen atoms on it.

The easiest way of spotting the difference is to draw or envision a line down the length of the double bond. Note, for cis and trans isomers the “same side” of the bond refers to the flat length of the bond. Recall, trans means “other side of” and cis means “on the same side as.”

Left: Trans Molecule
Right: Cis Molecule

For the leftmost molecule, the halogen (bromine) groups are on opposite sides of the dashed line, while in the example on the right, the halogen groups are on the same sides of the double bond.

Cis Trans Isomer Properties

Because of their “U” type shape, cis molecules do not pack as easily into solids as trans isomers do. This means that the intermolecular forces of cis isomers are not as strong. The shape also influences the polarity of the molecules. Refer to the table below for some differences in cis/trans molecule properties.

MoleculePolarityMelting PointBoiling PointSolubility in Neutral Solvent

For More Help, Watch our Interactive Video Explaining Alkene Isomers!

Further Reading

See the topics below to get a better understanding of additional isomer topics