What Is Cis Trans Isomerism Definition Conditions and Examples
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.
FAQs on Cis Trans Isomerism in Organic Chemistry
1. What is cis-trans isomerism in chemistry?
Cis-trans isomerism is a type of geometrical isomerism where compounds have the same molecular formula but differ in the spatial arrangement of groups around a rigid structure such as a double bond or ring.
- It occurs due to restricted rotation around a C=C double bond or within a cyclic compound.
- Cis isomer: similar or identical groups are on the same side.
- Trans isomer: similar or identical groups are on opposite sides.
- Example: In but-2-ene (C4H8), the methyl groups can be arranged as cis-but-2-ene or trans-but-2-ene.
2. What is the difference between cis and trans isomers?
The main difference between cis and trans isomers is the relative position of similar groups around a double bond or ring.
- Cis: Similar groups are on the same side of the double bond or ring.
- Trans: Similar groups are on opposite sides.
- Cis isomers usually have a higher boiling point due to polarity.
- Trans isomers are generally more stable because bulky groups are farther apart.
3. What conditions are necessary for cis-trans isomerism to occur?
Cis-trans isomerism occurs when there is restricted rotation and each carbon of a double bond is attached to two different groups.
- Presence of a C=C double bond or a ring structure.
- Each carbon of the double bond must have two different substituents.
- If one carbon has two identical groups, geometrical isomerism is not possible.
4. Why does a double bond cause cis-trans isomerism?
A C=C double bond causes cis-trans isomerism because it prevents free rotation around the bond.
- A double bond consists of one sigma (σ) bond and one pi (π) bond.
- The π bond locks the atoms in a fixed position.
- Breaking the π bond (which requires energy) is needed for rotation.
5. Can you give an example of cis-trans isomerism in alkenes?
A classic example of cis-trans isomerism in alkenes is but-2-ene (C4H8).
- Cis-but-2-ene: Both CH3 groups are on the same side of the C=C bond.
- Trans-but-2-ene: The CH3 groups are on opposite sides.
- Both have the same structural formula but different spatial arrangements.
6. Do cycloalkanes show cis-trans isomerism?
Yes, cycloalkanes can show cis-trans isomerism due to restricted rotation within the ring structure.
- The ring prevents free rotation of substituents.
- Cis: Substituents are on the same side of the ring plane.
- Trans: Substituents are on opposite sides of the ring plane.
- Example: 1,2-dimethylcyclohexane exists as cis and trans isomers.
7. How do you identify cis and trans isomers in a structure?
To identify cis and trans isomers, compare the positions of similar or priority groups around a double bond or ring.
- Step 1: Locate the C=C double bond or cyclic structure.
- Step 2: Ensure each carbon has two different substituents.
- Step 3: If similar groups are on the same side → cis.
- Step 4: If similar groups are on opposite sides → trans.
8. What is the difference between geometrical isomerism and cis-trans isomerism?
Cis-trans isomerism is a specific type of geometrical isomerism.
- Geometrical isomerism refers to isomers with different spatial arrangements due to restricted rotation.
- Cis-trans isomerism applies when identical groups are compared.
- For complex cases with different substituents, E–Z notation is preferred.
9. Which is more stable, cis or trans isomer?
In most cases, the trans isomer is more stable than the cis isomer.
- In trans isomers, bulky groups are farther apart, reducing steric repulsion.
- Cis isomers often have greater crowding on the same side.
- Example: Trans-but-2-ene is more stable than cis-but-2-ene.
10. Why are cis isomers often more polar than trans isomers?
Cis isomers are often more polar because their bond dipoles reinforce each other instead of canceling out.
- In cis isomers, polar bonds are on the same side, creating a net dipole moment.
- In trans isomers, dipoles often cancel due to symmetry.
- Higher polarity usually leads to a higher boiling point in cis isomers.
