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Isomerism is a very difficult topic in Chemistry because it always tends to confuse us between Cis and Trans types. While studying the isomers in chemistry, we came across different forms of isomers. Isomers are generally into two different groups – stereoisomers and constitutional isomers, but there are other sub-divisions of the isomers as well such as cis and trans isomers. In this article, we will be dealing with the difference between cis and trans isomers, and discussing some important questions.
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Key takeaways: Cis Trans Isomerism, isomerism, Organic and inorganic compounds, stereoisomerism, Geometric isomerism.
An Introduction to Cis and Trans Isomers
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The cis and trans isomerism is generally found in both the organic and inorganic compounds. The ‘cis’ and ‘trans’ are derived from the Latin word that refers to the side of and another side of respectively. Cis isomers are the isomers that have 2 similar atoms lying on the same side of the double bond in a molecule and the Trans isomers are the isomers that have 2 similar atoms lying on the opposite side of the double bond in a molecule. It is to be noted here that both the cis and Trans isomers have the same molecular formula but differ in a few aspects.
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What are Cis and Trans Form of Isomer?
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In stereoisomerism, the atoms that makeup isomers join up in the same order but have different spatial arrangements. Geometric isomerism is a kind of stereoisomerism. Geometric isomerism tends to occur when the rotation in a molecule is restricted. They generally consist of a carbon-carbon double bond.
Difference between Cis and Trans Isomers
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The basic difference between cis and trans isomers lies in the arrangement of the atoms in the double bonds. But it will be clearer to you when you will see them in tabulated form. Let us look at the table below:
| Cis | Trans |
|---|---|
| The molecules having the same connectivity of the atoms is referred to as the cis isomers. They constitute similar side groups which are placed on the same side of the double bond. | On the other hand, Trans isomer consists of molecules having similar side groups which are placed on the opposite side of the double bond. |
| The Cis isomers are generally polar in nature. | They are not much polar in nature. Many of the known trans isomers are non-polar molecules. |
| The cis isomers possess relatively lower melting points when compared to trans isomers because of the loosely packed molecules. | The melting points of the trans isomers are generally higher than that of the cis isomers because they are tightly packed molecules. |
| The cis isomers tend to possess a higher boiling point because of the presence of the strong attractive forces between the two atoms. | When compared to the cis isomer, the boiling point of the trans isomer is lower as there are no such strong attractive forces. |
| In the inert solvents, the cis isomers possess a strongly soluble content. | The solubility of trans isomers in neutral solvents is relatively smaller. |
| Both the form of cis isomers has a higher acid power. | It has lesser acidic strength as the protons in the trans molecule are not readily emitted. |
Boiling and Melting point of Cis and Trans Isomers
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- Boiling point: The cis- isomer has higher boiling point than the Trans- isomer of the same molecule. Take for e.g. the cis and trans isomer of 1,2-dichloroethane boiling point are 60°C and 48°C respectively. The reason behind this is, in the cis isomer, the heavy groups are on the same side of the double bond which in turn makes the cis isomer polar and results in dipole attraction between the molecules.
- Melting point: The Cis-isomer has lower melting point than the Trans-isomer. Take for e.g. the cis and trans isomer of 1,2-dichloroethane melting point are -80°C and -50°C respectively because the molecules of the Trans-isomers which is in straight chain are well packed than the molecules of the cis-isomers which is U-chained. Hence, the intermolecular forces are stronger in the trans- isomers when compared to the Cis-isomers.
Acidity of Cis and Tran Isomers
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The acidic strength of cis-isomers is higher than the Trans-isomers. The cis-isomers which are also called the maleic acid has greater acidic strength than the Trans-isomers which is also called as fumaric acid. This is because the maleic acid is able to form an intra-hydrogen bond. The intra-hydrogen bond forms because the maleic acid is able to lose its hydrogen ion (H+). Whereas, there is no such bond is formed in the fumaric acid.
Things to Remember
- The cis and trans isomerism is generally found in both the organic and inorganic compounds.
- The ‘cis’ and ‘trans’ are derived from the Latin word that refers to the side of and another side of respectively.
- Both the cis and Trans isomers have the same molecular formula but differ in a few aspects.
- In stereoisomerism, the atoms that makeup isomers join up in the same order but have different spatial arrangements. Geometric isomerism is a kind of stereoisomerism.
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Sample Questions
Ques. What are isomers? (2 marks)
Ans. The isomers refer to the molecules that possess the same molecular formula but different spatial arrangements of the atoms. It means that it excludes all the different arrangements that are just because of the molecule which are rotating as a whole unit around a particular bond. The structural isomerism can be defined as when the atoms that makes up the different isomers join together but in different order.
Ques. What are stereoisomers? (2 marks)
Ans. In stereoisomerism, the atoms that make up isomers join up in the same order, but have different spatial arrangements. There are generally 2 types of stereoisomers, namely
- Enantiomers – these are the stereoisomers that have mirror images, similar to the human hand.
- Diastereomers- These are the isomers other than the mirror images.
Ques. Explain how are the polar molecules are different from the non-polar molecule? (2 marks)
Ans. Polar molecules are the molecules in which two atoms do not share electrons equally in a covalent bond whereas Non-polar molecules are the molecules in which equal electron sharing take place in a covalent bond. They both also differ in symmetry as the polar molecules are asymmetrical in nature while the non-polar molecules are symmetrical in nature as there is no net dipole. The example of polar molecule is water and non-polar molecule is oil.
Ques. What are the difference between cis- isomers and trans-isomers. (5 marks)
Ans. The difference between cis and trans isomers can be listed as below:
| CIS | TRANS |
|---|---|
| The molecules having the same connectivity of the atoms is referred to as the cis isomers. They constitute similar side groups which are placed on the same side of the double bond. | On the other hand, Trans isomer consists of molecules having similar side groups which are placed on the opposite side of the double bond. |
| The Cis isomers are generally polar in nature. | They are not much polar in nature. Many of the known trans isomers are non-polar molecules. |
| The cis isomers possess relatively lower melting points when compared to trans isomers because of the loosely packed molecules. | The melting points of the trans isomers are generally higher than that of the cis isomers because they are tightly packed molecules. |
| The cis isomers tend to possess a higher boiling point because of the presence of the strong attractive forces between the two atoms. | When compared to the cis isomer, the boiling point of the trans isomer is lower as there are no such strong attractive forces. |
| In the inert solvents, the cis isomers possess a strongly soluble content. | The solubility of trans isomers in neutral solvents is relatively smaller. |
| Both the form of cis isomers has a higher acid power. | It has lesser acidic strength as the protons in the trans molecule are not readily emitted. |
Ques. why the cis isomer have higher boiling point than the Trans isomer? (2 marks)
Ans. The cis- isomer has higher boiling point than the Trans- isomer of the same molecule. The reason behind this is, in the cis isomer, the heavy groups are on the same side of the double bond which in turn makes the cis isomer polar and results in dipole attraction between the molecules.
For e.g. the cis and trans isomer of 1,2-dichloroethane boiling point are 60°C and 48°C respectively.
Ques. Why the cis-isomer has lower melting point than trans- isomer? (2 marks)
Ans. The Cis-isomer has lower melting point than the Trans-isomer because the molecules of the Trans-isomers which is in straight chain are well packed than the molecules of the cis-isomers which is U-chained. Hence, the intermolecular forces are stronger in the trans- isomers when compared to the Cis-isomers e.g. the cis and trans isomer of 1,2-dichloroethane melting point are -80°C and -50°C respectively.
Ques. Comment on the acidic strength of Cis and Trans- isomers. (2 marks)
Ans. The acidic strength of cis-isomers is higher than the Trans-isomers. The cis-isomers which are also called the maleic acid has greater acidic strength than the Trans-isomers which is also called as fumaric acid. This is because the maleic acid is able to form an intra-hydrogen bond. The intra-hydrogen bond forms because the maleic acid is able to lose its hydrogen ion (H+). Whereas, there is no such bond is formed in the fumaric acid.
Ques. Give an example of Cis and Trans- isomers in Alkenes. (2 marks)
Ans. Taking But-2-ene which is the most common example of geometric isomerism in Alkenes. Here, But-2-ene can show both the cis and trans-isomerism because of the restricted rotation around the double bond. The only difference lies in the placement of the methyl group as in cis-isomer, the two methyl groups are present on the same side of the double bond whereas in the Trans-isomers the two methyl groups are present on the opposite side of the double bond. Both these compounds possess different geometrical structures, physical and chemical properties.
Ques. How to get Geometric isomers? (2 marks)
Ans. There are few things to be considered to achieve the geometric isomers. These are as follows:
- The rotation of the compound must be restricted.
- It is necessary that there must be two different groups on the left- hand side of the carbon-carbon bond and similarly, two different groups on the right-hand side as well.
By achieving these two mentioned conditions we can easily get geometrical isomers of an organic or an inorganic compound.
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