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Cis Trans Isomerism is a kind of stereoisomerism observed in organic chemistry. Isomers are compounds, molecules or polyatomic ions that have identical molecular formulas but differ in their structure. The possibility of the presence of isomers is called Isomerism.
Two types of isomerism are structural isomerism and stereoisomerism. The structural isomerism can be further divided into positional, functional, group and chain isomerisms. While the stereoisomerism can be divided into conformational, configurational, optical and geometric isomerisms.
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Keyterms: Isomerism, Stereoisomerism, Organic chemistry, Isomers, Compounds, Molecules, Polyatomic ions, Atoms, Inorganic compounds, Geometric Isomerism
How to Identify Cis Trans Isomerism?
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This is a form of stereoisomerism that can be distinguished based on the position of isomers and hence can also be called Geometric Isomerism. Both organic and inorganic compounds exhibit cis-trans isomerism. In these cases, the number of atoms of each element in the molecules is the same. But the configuration of functional groups is different.
Though the molecular formula is identical, the compounds have different arrangements of atoms with distinct physical and chemical properties due to the orientation of functional groups. Based on the position, the compounds can be named accordingly.
Cis
Trans
Consider a compound of type abC = Cab can exist in two forms because of the rotation about the carbon double bond. Rotation is quite easy and possible in the case of single bonds between atoms. But the rotation in the case of double bonds is difficult. When the positions of atoms are fixed it results in Cis trans isomerism.
- Isomers that have similar functional groups on the same side of the bond is called Cis.
- Isomers that have similar functional groups on the opposite side of the bond are called Trans.
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Properties of Cis-Trans Isomerism
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The cis-trans isomer compounds may contain single, double or triple bonds with two functional groups. They may contain different physical and chemical properties. But not all the molecules exhibit cis-trans isomerism. Some of the conditions that may be applied to observe the cis-trans Isomerism are as follows:
- Presence of a restricted atom.
- This restricted atom must contain two functional groups individually.
- And both the restricted atoms must have one identical functional group.
| Compound | Cis trans isomerism |
|---|---|
| If two different atoms or groups are attached | Exists |
| If two similar atoms or groups are attached | Does not exist |
Limitations of Cis Trans Isomerism
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If all the restricted atoms have different functional groups then the cis-trans isomerism does not exist. For example, in the case of bdC = Cac. Here different functional groups a, b, c and d are present. There is no identical functional group. So, we cannot write the cis-trans isomerism.
Examples of Cis trans Isomerism
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In 1,2 dichloroethene the double bond between the carbon means the positions of functional groups either above or below the bond are important. Due to the rigid structure of compounds, the functional groups arranged in space have different configurations. The double bonds have restricted rotation. So the atoms stay in the same position frozen and are not easy to flip. With the double bond in between, we can comprehend that the compound remains similar.
The two chlorines lying on the same side of the isomer is called the cis isomer i.e., Cis 1,2 – dichloroethene. While the two chlorines on different sides of isomers are called the trans isomer i.e., Trans 1,2 – dichloroethene. Here the boiling point of cis isomer is 60.3°C and that of the trans isomer is 47.5°C.
In Organic Compounds
Organic Compounds
In the first figure, the compound has both the methyl groups on the same side and is called Cis-2-butene. Whereas in the second figure, the compound has methyl groups on different sides and is called trans-2-butene.
In Inorganic Compounds
Inorganic Compounds
This is the compound with the structural formula Pt(NH3)2Cl2. It consists of two ammonium functional groups. Due to the orientation of the functional groups the compounds have distinct physical and chemical properties.
In the first figure, the two ammonium functional groups are on different sides and are called trans platin or trans diamine dichloro platinum (II). Whereas in the second figure, the compound has ammonia groups on the same side and is called cis-platin or cis diamine dichloro platinum (II)
Things To Remember
- Isomers that have similar functional groups on the same side of the bond is called Cis.
- Isomers that have similar functional groups on different sides of the bond are called Trans.
- In alkenes, there are restricted atoms and they contain different functional groups individually and therefore they exhibit cis-trans isomerism.
- As trans isomers are non-polar, they are more stable.
- The increased stability in trans isomers is the reason for the higher boiling point.
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Sample Questions
Ques. Explain cis-trans isomerism with an example. (5 marks)
Ans. Cis trans Isomerism is a stereoisomerism where molecules having the same structural formula but with different orientations of the functional groups exhibit Isomerism. Due to this, the molecules have distinct physical and chemical properties.
When the identical functional groups are on the same side of a plane it is called Cis isomer. And when the identical functional groups are on the opposite side of the plane it is called Trans isomer.
Generally, this Isomerism can be observed in the molecules having double or triple bonds. It is due to a restricted rotation that the positions of atoms are fixed. So, cis-trans isomerism is exhibited. Both in organic and inorganic compounds cis-trans isomerism exists.
In pentene, the two Carbon atoms are bonded with different atoms and groups. So, the compound exhibits cis-trans isomerism.
Ques. Why do alkenes exhibit cis-trans isomerism? (3 marks)
Ans. Alkenes consist of different substitutes at each end of C=C. The double bond between the carbon atoms is restricted from rotation. That means the atoms cannot be flipped or interconverted. The conditions of Cis trans isomerism are:
- Presence of a restricted atom.
- This restricted atom must contain two functional groups individually.
- And both the restricted atoms must have one identical functional group.
In alkenes, there are restricted atoms and they contain different functional groups individually. So, alkenes always exhibit cis-trans isomerism.
Ques. Why is cis-trans isomerism important? (2 marks)
Ans. The properties of important industrial materials can be analyzed and studied with the concept of cis-trans isomerism. There exists high practical importance to study the physical and chemical properties of various organic and inorganic compounds. However, isomerization acts as a way of obtaining desired materials with desired characteristics.
Ques. Is cis more stable than trans? (2 marks)
Ans. In the trans isomer, the functional groups are attached on the opposite side. Hence the trans molecule is non-polar. While the functional groups are attached on the same side. So, it is polar. Hence the trans isomer is more stable than the cis isomer.
Ques. Why trans has more melting point than cis? (2 marks)
Ans. The attractive forces between the atoms are less and reduced due to the less polarity of the molecules. The decreased forces lead to a lower boiling point. The trans isomers are tightly packed with higher symmetry. Hence the attractive forces between the atoms are more. So, the trans isomers have a higher melting point than the cis isomers.
Ques. Does methyl propene consist of cis or trans isomer? (2 marks)
Ans. The methyl propene does not consist of cis or trans isomerism. Other than this, the alkenes with C=CR2 units comprising of the same R groups do not exhibit cis-trans isomerism.
Ques. Differentiate between cis and trans isomers? (3 marks)
Ans.
| Cis Isomer | Trans Isomer |
|---|---|
| The molecules having functional groups on the same side of the double bond is called Cis Isomer. | The molecules having functional groups on the opposite side of the double bond is called Trans isomer. |
| They are polar molecules. | They are non-polar molecules. |
| Cis isomers have a lower melting point. | Trans isomers have a higher melting point. |
| Have strongly soluble content in inert solvents. | The solubility of trans isomers is not so strong and neutral. |
| Have high acidic strength. | Have less acidic strength. |
Ques. Draw the cis- and trans-structures for hex-2-ene. Which isomer will have higher b.p. and why? (2 marks)
Ans. The structures of cis- and trans-isomer of hex-2-ene are:
The boiling point of a molecule depends upon dipole-dipole interactions. Since cis-isomer has a higher dipole moment, therefore, it has a higher boiling point.
Ques. Draw the structures of cis- and trans-isomers of the following compounds. Also, write their IUPAC names.(i) CHCl—CHCl (ii) C2HC(CH3)=C(CH3)C2H5 (2 marks)
Ans. 
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