Cis-Trans Isomers: Organic & Inorganic Compounds

In Chemistry, when we are studying isomers, we will definitely come across various types of isomers. They are mainly divided into two groups namely constitutional isomers and stereoisomers. There are more sub-type of isomers. As such, cis and trans isomers are such type of isomers which we are going to learn here.

Read Also: Elimination Reaction

What are Isomers?

Isomers are molecules, compounds, or polyatomic ions which have an identical molecular formula with the same number of atoms but differ in the way in which they are arranged (i.e., differ in their structure from each other). The presence of isomers is called Isomerism. There are two types of isomers, that can further be divided:

1. Structural isomers
2. Stereoisomers

What are Cis-Trans Isomers?

Cis-Trans isomers are Stereoisomers that exhibit stereoisomerism. Cis-trans isomerism is also known as geometrical isomerism or configurational isomerism. The words “Cis” and “Trans” mean “This side of” and “other side of” in Latin context. Isomers that have their functional groups on the same side are called Cis-isomers and isomers that have their functional groups on the opposite sides are called Trans-isomers.

Structure of Cis-Trans Isomers:

Let R, H be two different functional groups. Consider a compound RHC=CHR. If observed in the below image Cis isomer has the functional groups R and H on the same side. Whereas in Trans isomer the functional groups are on the opposite side of the Carbon bond.

Examples of Cis-Trans Isomers:

Some examples of Cis-Trans Isomerism in organic and inorganic compounds is given below

Organic Compounds: The presence of a double or a triple bond in a molecule restricts the bond rotation within the molecule.

Inorganic Compounds:  Many diazines and diphosphates exhibit cis-trans isomerism. Based on the positions of ligands, coordinate complexes that have square planar or octahedral geometries also exhibit cis-trans-isomerism. It can be observed that similar ligands are positioned on the same side in cisplatin and on the opposite side in transplatin.

Properties of Cis-Trans Isomers:

• Cis Isomers are mostly much reactive compared to Trans isomers.
• Cis Isomer can reduce alkenes and alkynes to alkanes.
• Cis Isomer can occur in organic as well as inorganic compounds
• A molecule is said to be exhibiting cis-trans isomers only if all these three rules are satisfied:

1. A restricted atom is present.
2. Presence of two different functional groups individually in both the restricted atoms.
3. At least one functional group is commonly present in both the restricted atoms.

• Alkenes with more than two different substituents cannot use Cis-Trans notation.

Read Also: Class 12 Carbon and Its Compounds

Things to Remember:

• Cis isomers have functional groups present on the same side of the bond whereas trans isomers have functional groups present on the opposite side of the bond.
• Cis isomers have a higher boiling point because of the presence of strong attractive forces between the atoms of the cis isomer.
• Trans isomers have a lower boiling point because of the absence of strong attractive forces between the atoms of Trans-isomer.
• Cis isomers have a low melting point compared to trans isomers because the molecules are loosely packed in Cis isomers.
• Trans isomers have a high melting point compared to cis isomers because the molecules are tightly packed in Cis isomers.