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Cis-trans alkene or Cis-trans isomerism, is also popular with the name of Configurational Isomer or Geometrical Isomer. It is a type of isomerism involving carbon atom arrangement within the molecules.
- Trans Alkene is one category of Configurational Isomer or Cis-Trans Isomer.
- It is a type of alkene in which two substituent groups of atoms lie on the opposite side of the double bond.
- Trans Alkene is the most stable category of alkene in comparison to Cis Alkene.
- Cis Trans alkene can be differentiated on the ground of boiling points, dipole movement and melting points.
- Hydrogen atoms can also be used for the identification of trans alkene.
Read More: Alkynes
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Key Terms: Trans Alkene, Cis Alkene, Isomerism, Alkene, Boiling Points, Melting Points, Sodium, Hydrogenation, Molecules, Double bond
What is Trans Alkene
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Trans Alkene is an alkene in which functional groups are attached to the different sides of a double bond.
- In trans alkene, the molecular formula of the required structure is the same, but the spatial arrangement is different.
- It is used in organic molecules and inorganic coordination complexes.
- In this category of alkene substituents, groups are facing away from each other.
- The orientation of the group attached to the double bond is fixed.
- Cis Trans alkene also exists in the form of a ring structure or cyclic compound.
Trans 2-Pentene
Structure of Trans Alkene
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In trans alkene, the functional group lies on the opposite side of the double bond. It can be explained with the help of below example:
Structure of Trans Alkene
In the above diagram, cis and trans alkene is possible if R1 is not equivalent to R2 and similarly R3 is not equivalent to R4.
Read More: IUPAC Nomenclature of Organic Compounds
Characteristics of Trans Alkene
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Trans Alkene is different from other alkene due to differences in physical properties, dipole movements and stability of molecules.
Dipole Movement of Trans Alkene
Trans Alkene is the most stable class of alkene. It has zero dipole moment as the bond moments between the isomers are equal and opposite.
- This type of trans alkene in which the net dipole moment is zero is called symmetrical trans alkene.
- The result is different in the case of asymmetrical trans alkene.
- In asymmetrical trans alkene, the net dipole movement has some value as the isomers are not equal and opposite.
Read More: Addition Reaction of Alkynes
Boiling Point of Trans Alkene
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In trans alkene, substituent groups are attached on different sides of the double bond. The properties of trans alkene are based on boiling points, which depend on the attractive forces between the molecules.
- The boiling point increases the size of the substituent group attached to the carbon atoms.
- In the case of trans alkene, the boiling point is less than its counterpart, cis alkene.
- The reason is the polarity of trans alkene, which is less than cis alkene.
- Polarity basically increases the intermolecular force between the functional groups, thus increasing the boiling points.
The boiling point increases every time a CH2 group is added to the structure.
Stability of Trans Alkene
Trans alkene belongs to the class of stable alkene as both the functional groups are facing away from each other.
- In trans alkene, less heat is combusted, indicating higher thermodynamical stability.
- There is less steric interaction between the substituents in the case of trans alkene.
Melting Point of Trans Alkene
The final factor that helps in deciding the characteristics of trans alkene is its melting point.
- The melting point depends upon the polarity and structure of the molecule.
- In the case of trans alkene, the melting point is high in comparison to cis alkene.
- The polarity of trans alkene is low.
- The structure of the molecule is more symmetrical in the case of trans alkene.
Hydrogenation of Alkynes to Trans Alkene
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Trans alkene can easily be created with the help of alkynes. The most common method to obtain trans alkene is through the hydrogenation of alkynes.
- In this process, sodium atoms act as a reducing agent due to the presence of 3s electrons.
- It is also known as the dissolving metal reduction.
- When alkyne bonds react with the sodium metal dissolved in the ammonia, the addition of hydrogen takes place.
- This addition of anti-hydrogen to the alkyne bond results in the formation of trans alkene.
Comparison between the Trans Alkene and Cis Alkene
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The difference between the Trans Alkene and Cis Alkene is provided in the table below:
| Trans Alkene | Cis Alkene |
|---|---|
| In this substituent groups are attached on different sides of the double bond. | In this substituent groups are attached on the same sides of the double bond. |
| It has a high melting point as the molecules are tightly packed. | Cis alkene has a low melting point as molecules are loosely packed. |
| Trans Alkene has a low boiling point as the attractive force between molecules is weak. | Cis Alkene has a high boiling point as it possesses a strong attractive force between molecules. |
| The polarity of trans alkene is less as it has many non-polar molecules. | The polarity of cis alkene is more in comparison to trans alkene. |
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Things to Remember
- Trans Alkene is a type of alkene in which functional atoms are present on the different sides of the double bond.
- It can easily be created with the hydrogenation of alkynes.
- The molecular formula of trans alkene is similar, but the structural formula is different.
- It depends upon the structure's boiling point, melting point and stability.
- The trans alkene belongs to the class of stable alkene.
- The polarity of trans alkene is less in comparison to cis alkene.
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Sample Questions
Ques. Explain the meaning of trans alkene with the help of an example. (3 marks)
Ans. Trans Alkene is a type of alkene in which substituent groups are present on the different sides of the double bond of carbon atoms. This alkene class is considered more stable than its counterpart, cis alkene.
The most common example of Trans Alkene is as follows:
Ques. What is alkene, and what are its types? (5 marks)
Ans. Alkene is a type of unsaturated hydrocarbon which consists of at least one carbon-carbon double bond. It is also popular with the name olefins. This is the most reactive class of hydrocarbon due to the presence of carbon double bonds. If it has more than four carbon atoms, it can be used as structural isomers.
Alkene is of two types which are as follows:
Cis Alkene: It is a type of alkene in which functional groups are attached to the same side of a double bond.
Trans Alkene: It is a type of alkene in which functional groups are present on the different sides of a double bond.
Ques. What is the difference between Cis Alkene and Trans Alkene? (4 marks)
Ans. The table below showcases the difference between trans alkene and cis alkene
| Trans Alkene | Cis Alkene |
|---|---|
| Trans Alkene is an alkene in which groups are attached to different sides of a double bond. | Cis Alkene is an alkene in which groups of atoms are attached on the similar side of the double bond. |
| It has a high melting point and low boiling point. | Cis alkene has a low melting point and high boiling points. |
| The acidic strength of trans alkene is less. | Cis Alkene has high acidic strength. |
| The solubility power of trans alkene is less in neutral solvents. | The solubility power of trans alkene is more in neutral solvents. |
Ques. How can trans alkene can be formed by the process of hydrogenation? (3 marks)
Ans. Hydrogenation of alkynes takes place when sodium or lithium atoms dissolved in ammonia react with the pi bond of alkynes.
The process of synthesis of alkynes to form trans alkene is as follows:
- In this process, sodium atoms act as a reducing agent as they can easily give up their electrons.
- A radical anion is created when sodium atoms donate their first electron to the alkyne.
- In the next step, another sodium electron is donated to the alkyne to form a vinyl anion.
- This trans vinyl is strong and rapidly converts into the required trans alkene.
Ques. What are the characteristics of trans alkene? (2 marks )
Ans. Trans alkene is defined as an alkene in which a group of atoms are present on the opposite side of double bonds. The characteristics of trans alkene are as follows:
- Dipole moment of trans alkene
- Melting and boiling point of trans alkene
- Stability of the trans alkene
- The acidic nature of the trans alkene
Ques. Give the chemical reaction for the conversion of alkyne to trans alkene. (1 mark)
Ans. The chemical reactions for the conversion of alkyne to trans alkene are as follows:
CH3C≡CH3 + Na / NH3 → CH3CH=CHH3
Ques. How does melting point affect the characteristics of trans alkene? (3 marks)
Ans. The melting point of trans alkene is high as the molecules of the required compound are packed close to each other. The melting point of any compound depends upon the symmetrical structure and polarity of the molecule. In the case of trans alkene, its polarity is less due to the presence of a trans molecule. The symmetrical structure of trans alkene is more strong in comparison to its counterpart cis alkene.
Ques. What is a configurational isomer? (2 marks)
Ans. Configurational Isomer also known as geometric isomer is a type of isomer in which the molecular formula of the compound is different but the structural formula of the compound is the same because of the spatial arrangement of atoms within the molecules.
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