Cis Trans Isomerism

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Cis Trans Isomers

Isomers are molecules with identical molecular formulas. It means that the number of atoms of each element in isomers is the same. Isomers differ in their structural or organizational pattern. They have their distinct arrangement in space. Isomers may look identical but exhibit different physical and chemical properties. 

Cis trans isomers also have the same number of atoms of each element. Cis and trans is a type of isomerism that exists, differentiating the isomers based on their position, also known as geometric isomerism. It exists in both organic and inorganic compounds. Cis trans isomerism is an example of stereoisomerism, which means that each element's number of atoms in the molecule is the same. Still, the functional group's configuration is different. They are rotated differently when observed in a three-dimensional space. Trans isomers are more stable than their cis counterparts. They are more stable due to their superior thermochemical stability owing to the lower heat emission during combustion. However, there are some exceptions where cis isomers are more stable than trans-isomers.


What is Cis Trans Isomerism?

Cis trans isomerism or geometrical isomerism is also known as configurational isomerism. Cis means "on this side of," and trans means "on the other side of." In the cis orientation, the functional groups are on the same side of the carbon-carbon double bond. In trans orientation, the functional groups are present on opposite sides of the double bond. This difference in orientation brings about a difference in the physical and chemical properties of the isomers. 

While naming the compound, cis and trans prefixes are added to the compound name. An important point to observe is that cis and trans isomers exist in both organic and inorganic chemistry. This isomerism exists in the alkene compounds. These compounds have a carbon-carbon double bond in them. Cis and trans isomerism are tricky to comprehend. The compound may seem similar if you rotate them, but an important point to remember is that only carbon-carbon single bonds are freely rotatable, so if there is a double bond between two carbon atoms and functional groups attached to them. The compounds will differ based on the position of the functional group.

Suppose two methyl compounds are attached to 2 carbon atoms sharing a double bond. The position of the methyl functional group will tell you how the compound will be named. If the methyl functional groups are placed on the same side, it will be named cis- and if the methyl functional group lies on the opposite sides of the plane, it will be named trans-.  


Cis Trans Isomers Examples

Cis and trans isomerism exist in both organic and inorganic compounds. These are stereoisomers in which the functional group's position is rotated around the alkene bond. Depending upon these functional groups' positions around the carbon-carbon double bond, the molecules with the same number of atoms of each element are named differently. Cis isomers have functional groups on the same side, and trans isomers have functional groups present on the opposite sides. Below are some common examples of cis and trans isomerism.


Cis Trans Isomerism in Organic Compounds 

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Here two methyl groups are attached to the carbon atoms. The number of atoms of the elements in both compounds is the same. They have similar molecular formulas.

C4H8 - 2 butene ( 2 for the position of the carbon-carbon double bond and ene for the presence of a double bond).

However, the difference here is that the position of the methyl functional group is on the same side in the case of cis-2-butene and on the opposite sides in the case of trans-2-butene.


Cis Trans Isomerism in Inorganic Compounds 

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Here, two ammonium functional groups are attached to a single platinum atom. 

The compound has a similar molecular formula- Pt(NH3)2Cl2.

When the two NH3 groups are on the same side, the compound is named cisplatin and when they are present on two different sides, the compound is called Transplatin.

Here the difference in the position of the Ammonium functional group makes it a compound with different physical and chemical properties. Cisplatin is commonly used in chemotherapy but transplatin has no useful anticancer activity. This goes to solidify the fact that change in the configuration of the compound brings about significant changes in its characteristics.

FAQ (Frequently Asked Questions)

Question 1. What are Stereoisomers?

Ans: Stereoisomers differ in the arrangement of the elements in the space. The number of atoms of each element remains the same in these isomers, but the functional groups' configuration is different. The bonds between these elements are also the same, but these elements' position in space differs. The difference in position may seem like it does not bring any significant change but the internal energy of the compound changes because it is dependent upon the angles and distances between different bonds. Stereoisomers exist in 2 ways- Diastereomers and Enantiomers. Enantiomers are optical isomers, i.e., they are related to each other by mirror images. Diastereoisomers do not show optical isomerism but exhibit other isomerisms like cis-trans, e/z, etc.

Question 2. Explain Cis-trans Isomerism with the help of an example.

Ans: Cis trans isomerism is geometrical isomerism, where compounds with the same molecular formula have a different configuration. This difference in configuration results in different physical and chemical properties. Even though there is a slight difference, the changes due to change in the compound's configuration affect the internal energy of the compound, which is dependent on the angles of the bonds formed and the distance between the atoms of the elements. 

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The first compound is cis-2-pentene, and the second one is trans-2-pentene. Here the methyl functional group attached on the same side in the first compound is visible. The second compound, on the other hand, has methyl functional groups on the opposite sides.