Cis-trans isomers exhibit a type of stereoisomerism where the atoms have different spatial arrangements in three-dimensional space. In the field of organic chemistry, cis isomers contain functional groups on the same side of the carbon chain whereas the functional groups are on opposite sides in trans isomers.
This type of isomerism can arise in both organic and inorganic molecules. The prefix “cis” and “trans” have Latin roots and can be translated as “this side of” and “other side of” respectively. Some coordination complexes have cis-trans isomers as well.
Differences in Properties
In many cases, the cis-trans isomers of a compound have different physical properties. These differences can arise due to the difference in the dipole moment of the molecule or due to the difference in the spatial arrangement of atoms. Some of these differences are highlighted below.
- The cis isomer of pent-2-ene has a boiling point of 37oC but the boiling point of the trans isomer is 36o The difference is small because the bond polarity is low.
- Due to the polar nature of the bonds in 1,2-dichloroethylene, the boiling point of the cis isomer is 60.3oC whereas that of the trans isomer is 47.5oC (the C-Cl dipole moments in the trans isomer cancel each other out).
- The cis and trans isomers of butenedioic acid display very different reactivities, which can be attributed to the difference in their properties. Maleic acid is the cis isomer and fumaric acid is the trans isomer.
- Elaidic acid and oleic acid are cis-trans isomers. The former is solid at room temperature (melting point = 43oC) and the latter is found to be liquid, with a melting point of 13.4o
Thus, it can be noted that trans isomers generally have higher melting points than their cis counterparts. They also tend to have lower solubility in solvents that are inert in nature. It has been suggested that the densities of trans isomers tend to be lower than that of cis isomers.
Generally, it can be observed that the individual bond dipole moments in trans isomers cancel each other out since they are on opposite sides. The cis isomers of acyclic systems tend to be less stable than the trans variation, which can be explained by the steric interaction of the substituents.
Examples of Cis-Trans Isomers
Some examples of cis-trans isomers are provided in this subsection along with illustrations.
The presence of a double or triple bond restricts the bond rotation within a molecule, which can lead to cis-trans isomerism. This type of isomerism can be observed in the organic compound but-2-ene.
The structures of the cis and trans isomers of but-2-ene have been illustrated above.
Many diazenes and diphosphenes are known to have cis-trans isomers. Coordination complexes having square planar or octahedral geometries also display cis-trans isomerism based on the position of the ligands. The isomers of the coordination compound Pt(NH3)2Cl2 are illustrated below.
It can be observed that similar ligands are positioned near each other in the cis isomer but they are positioned on opposite sides in the corresponding trans isomer. To learn about cis-trans isomers and other types of isomerism, register with BYJU’S and download the mobile application on your smartphone.