What is the Difference Between Symmetrical and Unsymmetrical Alkenes

The key difference between symmetrical and unsymmetrical alkenes is that symmetrical alkenes have doubly bonded carbon atoms bearing the same ligands, whereas unsymmetrical alkenes have doubly bonded carbon atoms bearing different ligands.

Alkenes are organic compounds composed of one or more carbon-carbon double bonds. These are types of hydrocarbons because alkene contains only carbon and hydrogen atoms. Often, we call them olefins. We can identify two major types of alkenes as terminal and internal alkenes according to the location of the double bond in the hydrocarbon chain. However, we can also classify alkenes into symmetrical and unsymmetrical alkenes depending on the effect of the double bond on the symmetry of the organic compound.

CONTENTS

1. Overview and Key Difference
2. What are Symmetrical Alkenes 
3. What are Unsymmetrical Alkenes 
4. Symmetrical vs Unsymmetrical Alkenes in Tabular Form
5. Summary – Symmetrical vs Unsymmetrical Alkenes

What are Symmetrical Alkenes?

Symmetrical alkenes are a type of alkene hydrocarbons having similar ligands attached to the doubly bonded carbon atoms. Therefore, we can name the carbon-carbon double bond in this type of chemical compound as a symmetrical double bond. Here, the ligands that are attached to the carbon atoms may or may not be identical. Let us consider an example to understand this phenomenon.

The above image shows two alkene compounds that are symmetrical. These are cis-trans isomers, and they have doubly bonded carbon atoms that are attached with a methyl group and a hydrogen atom at each carbon atom. These are symmetrical alkenes because they have similar ligands attached to each double-bonded carbon atom.

What are Unsymmetrical Alkenes?

Unsymmetrical alkenes are a type of alkene hydrocarbons having different ligands attached to doubly bonded carbon atoms. Therefore, we can name the carbon-carbon double bond in this type of chemical compound as an unsymmetrical double bond. In these molecules, the ligands that are attached to the carbon atoms are essentially non-identical. Let us consider some examples to understand this phenomenon.

In figure 2, there is a methyl group and a hydrogen atom attached to the right-side carbon atom and two hydrogen atoms attached to the left side carbon atom. The chemical compound becomes unsymmetrical because there are different ligands attached to each carbon atom.

In figure 3, the reactant is an unsymmetrical alkene compound. It has two doubly bonded carbon atoms attached to different ligands; one carbon atom is attached to a hydrogen atom and an adjacent carbon atom of the ring structure, whereas the other carbon atom is attached to a methyl group and an adjacent carbon atom of the ring structure.

What is the Difference Between Symmetrical and Unsymmetrical Alkenes?

Alkenes can be divided into two groups as symmetrical and unsymmetrical alkenes according to the symmetry of the molecule. The key difference between symmetrical and unsymmetrical alkenes is that symmetrical alkenes have doubly bonded carbon atoms bearing the same ligands, whereas unsymmetrical alkenes have doubly bonded carbon atoms bearing different ligands.

The following figure presents the difference between symmetrical and unsymmetrical alkenes in tabular form.

Summary – Symmetrical vs Unsymmetrical Alkenes

Alkenes are hydrocarbon compounds. They contain at least one carbon-carbon double bond. The key difference between symmetrical and unsymmetrical alkenes is that symmetrical alkenes have doubly bonded carbon atoms bearing the same ligands, whereas unsymmetrical alkenes have doubly bonded carbon atoms bearing different ligands.

Reference:

1. “Symmetrical Alkene.” Chemistry LibreTexts, Libretexts, 24 Aug. 2020.
2. “Unsymmetrical Alkene.” Chemistry LibreTexts, Libretexts, 24 Aug. 2020.

Image Courtesy:

1. “cis and trans 2-Butene” By HGTCChem (CC BY 2.0) via Flickr
2. “Propene Formula V.1” By Ju (CC0) via Commons Wikimedia