The key difference between cis and trans isomers is that the cis isomer has same atoms on the same side of the double bond whereas the trans isomer has two of the same atoms on the opposite sides of the double bond.
Isomers are different compounds with the same molecular formula. There are various types of isomers. But, we can mainly divide the isomers into two groups as constitutional isomers and stereoisomers. Here, the constitutional isomers are isomers where the connectivity of atoms differs in molecules. In stereoisomers, atoms connect in the same sequence, unlike constitutional isomers. Therefore, stereoisomers differ only in the arrangement of their atoms in space. Also, stereoisomers are of two types; enantiomers and diastereomers. Furthermore, cis and trans isomers are the two forms of diastereomers.
CONTENTS
1. Overview and Key Difference
2. What are Cis Isomers
3. What are Trans Isomers
4. Side by Side Comparison – Cis vs Trans Isomers in Tabular Form
5. Summary
What are Cis Isomers?
The molecule in which two of the same atoms are on the same side of the double bond is a cis isomer. The cis isomer has a higher boiling point compared to the trans isomer. The reason for this is the stronger intermolecular forces in cis isomers.
Figure 01: Cis Isomer of Butene
For example, in 1,2-dichloroethane, when the molecule is cis, two more electronegative chlorine atoms are on one side of the molecule. Because of this, that side of the molecule will have a slight negative charge while the other side will have a slight positive charge. Therefore, the molecule becomes a polar and dipole-dipole interaction may occur between molecules. These extra intermolecular forces in cis isomer give it a higher boiling point compared to trans isomers.
What are Trans Isomers?
A molecule with two of the same atoms in the opposite side of the double bond is the trans isomer. Trans isomers will have lower boiling points because although there is a charge separation, the overall molecule becomes non-polar.
Figure 02: Trans Isomer of Butene
Nevertheless, trans isomers have a higher melting point. Trans isomers have a straighter shape, and they pack well. Therefore, it requires higher energy to melt the molecule which gives it a higher melting point.
What is the Difference Between Cis and Trans Isomers?
The molecule in which two of the same atoms are on the same side of the double bond is known as the cis isomer. On the other hand, a molecule with two of the same atoms on the opposite sides of the double bond is known as the Trans isomer. Therefore, this is the key difference between cis and trans isomers. Also, by comparing their properties, we can identify some difference between cis and trans isomers. The cis isomers are polar while the trans isomers are comparatively non-polar. Therefore, the cis isomer has the higher boiling point compared to that in trans isomer. However, trans isomers have a higher melting point; in contrast, cis molecules have a lower melting point. This is mainly because, the trans molecules are packed well than cis molecules.
The below infographic presents the difference between cis and trans isomers in tabular form.
Summary – Cis vs Trans Isomers
In summary, the cis and trans isomers are diastereomers that come under the stereoisomers. And, the key difference between cis and trans isomers is that the cis isomer has same atoms on the same side of the double bond whereas the trans isomer has two of the same atoms on the opposite sides of the double bond.
Reference:
1. “Cis–Trans Isomerism.” Wikipedia, Wikimedia Foundation, 11 June 2018. Available here
Image Courtesy:
1.”Cis-2-butene”By JaGa – Own work, (CC BY-SA 3.0) via Commons Wikimedia
2.”Trans-2-butene”By JaGa – Own work, (CC BY-SA 3.0) via Commons Wikimedia