What is the Difference Between Symmetrical and Unsymmetrical Alkene

Symmetrical and unsymmetrical alkenes are two classifications based on the arrangement of substituent groups around the double bond. This structural difference between symmetrical and unsymmetrical alkene can influence the chemical reactivity, stereochemistry, and physical properties of the molecules.

What is the difference between symmetrical and unsymmetrical alkene? Symmetrical alkene molecules have identical substituents on both sides of the double bond, while unsymmetrical alkene molecules have different substituents on each side of the double bond.

Key Areas Covered

1. What is Symmetrical Alkene
      – Definition, Features
2. What is Unsymmetrical Alkene
      – Definition, Features
3. Similarities Between Symmetrical and Unsymmetrical Alkene
      – Outline of Common Features
4. Difference Between Symmetrical and Unsymmetrical Alkene
      – Comparison of Key Differences
5. FAQ: Symmetrical and Unsymmetrical Alkene
      – Answers to Frequently Asked Questions

Key Terms

Symmetrical Alkene, Unsymmetrical Alkene

What are Symmetrical Alkenes

A symmetrical alkene is a type of organic compound characterized by a carbon-carbon double bond where the substituents on each carbon atom of the double bond are identical. This means that the two carbon atoms bonded by the double bond have the same groups attached to them. Symmetrical alkenes are named according to the longest carbon chain containing the double bond, with the suffix “-ene” denoting the presence of the double bond.

In a symmetrical alkene, the double bond does not introduce any chirality or asymmetry into the molecule because the groups attached to each carbon of the double bond are the same. This results in a symmetrical distribution of electron density around the double bond, leading to similar reactivity on both ends of the double bond.

One of the key characteristics of symmetrical alkenes is their tendency to undergo addition reactions across the double bond. This is because the electron density in the π bond is evenly distributed, making both carbon atoms equally susceptible to electrophilic attack. Common addition reactions involving symmetrical alkenes include hydrogenation, halogenation, and hydrohalogenation.

Examples of symmetrical alkenes include ethene, propene, and butene. In these molecules, each carbon atom participating in the double bond is bonded to the same number and type of atoms or groups, resulting in symmetry about the double bond axis.

What are Unsymmetrical Alkenes

Unsymmetrical alkenes are a class of organic compounds characterized by the presence of a double bond between two carbon atoms where each carbon is bonded to different substituents. In other words, the groups attached to the carbon atoms on either side of the double bond are not identical. This lack of symmetry gives unsymmetrical alkenes unique properties and reactivities compared to their symmetrical counterparts.

One of the defining features of unsymmetrical alkenes is their ability to undergo various chemical reactions, such as addition reactions, in a regioselective manner. Regioselectivity refers to the preference of a reaction to occur at one site over another within a molecule. Due to the different electronic and steric effects of the substituents, the double bond in unsymmetrical alkenes can react selectively with certain reagents at one carbon atom over the other, leading to the formation of specific products.

Furthermore, unsymmetrical alkenes exhibit stereoisomerism, wherein they can exist as geometric isomers known as cis and trans isomers. Cis isomers have similar substituents located on the same side of the double bond, while trans isomers have them on opposite sides. This geometric difference can significantly influence the physical and chemical properties of these compounds, including their stability, reactivity, and biological activity.

Similarities Between Symmetrical and Unsymmetrical Alkene

1. Both types of alkenes exhibit similar reactivity patterns in chemical reactions involving the addition of the double bond.
2. Symmetric and unsymmetric alkenes have similar physical properties.

Difference Between Symmetrical and Unsymmetrical Alkene

Definition

Symmetrical alkenes are a class of organic compounds characterized by a carbon-carbon double bond where the substituents on each carbon atom of the double bond are identical. Unsymmetrical alkenes are a class of organic compounds characterized by the presence of a double bond between two carbon atoms where each carbon is bonded to different substituents.

Substituents

While symmetric alkenes have identical substituents on each side of the double bond, unsymmetric alkenes have different substituents on each side.

Nomenclature

Symmetric alkenes are named using the prefix “cis” or “trans” to denote the spatial arrangement of substituents, while unsymmetric alkenes are named based on the IUPAC system, which prioritizes substituents alphabetically to determine the main chain and assigns E or Z configuration to indicate the arrangement of substituents around the double bond.

Physical Properties

Symmetric alkenes tend to have more predictable physical properties due to their symmetric structure, while unsymmetric alkenes may exhibit varied physical properties depending on the nature of their substituents.

Conclusion 

In conclusion, understanding the difference between symmetrical and unsymmetrical alkenes illuminates the intricate dynamics of organic chemistry. Symmetrical alkenes, with their balanced molecular structures, typically exhibit greater stability, whereas unsymmetrical alkenes often showcase distinctive reactivity patterns due to their asymmetric arrangements. Recognizing these differences enables chemists to make informed decisions when designing synthetic routes or predicting the outcomes of chemical reactions, ultimately advancing the field of organic chemistry.

FAQ: Symmetrical and Unsymmetrical Alkene

1. Is ethene a symmetrical alkene?

Yes, ethene is a symmetrical alkene due to its molecular structure. The two carbon atoms in ethene are identical, each bonded to two hydrogen atoms, leading to symmetry across the molecule.

2. Is butene symmetrical?

Butene is not inherently symmetrical, but trans-2-butene exhibits a symmetrical structure due to its arrangement of substituents.

3. What is the difference between symmetrical and unsymmetrical reagents?

Symmetrical reagents possess identical or similar functional groups on both sides of a reaction, resulting in a balanced or symmetric structure. In contrast, unsymmetrical reagents contain different functional groups or atoms on each side, leading to an asymmetric or unbalanced molecular structure.

4. Are symmetrical alkenes more stable? 

Symmetrical alkenes are generally more stable than unsymmetrical alkenes. This is due to the balanced distribution of substituent groups around the double bond in symmetrical alkenes, which minimizes steric hindrance and strain within the molecule. 

Reference:

1. Gunawardena, Gamini. “Unsymmetrical Alkene.” LibreTexts. Chemistry.
2. Gunawardena, Gamini. “Symmetrical Alkene.” LibreTexts. Chemistry.

Image Courtesy:

1. “Trans-2-Butene Formula V.1” By Jü – Own work (CC0) via Commons Wikimedia