Geometric Isomerism and Cis/Trans Isomers

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. They are important building blocks in organic chemistry and have a wide range of applications in industry and daily life. One of the most important characteristics of alkenes is their ability to exhibit geometric isomerism, which arises due to the restricted rotation about the carbon-carbon double bond. In this lesson, we will discuss the concept of geometric isomerism in alkenes and how it leads to the formation of cis/trans isomers.

Geometric Isomerism in Alkenes:

In alkenes, the carbon atoms in the double bond are sp² hybridized and the double bond is formed by the overlap of one sp² hybrid orbital from each carbon atom with a p orbital. The resulting π bond restricts the rotation about the carbon-carbon double bond. As a result, different spatial arrangements of the substituents on each carbon atom are possible, leading to the formation of different isomers. Cis/Trans Isomers:

The most common type of geometric isomerism in alkenes is the cis/trans isomerism. In cis/trans isomers, the substituents on each carbon atom of the double bond are arranged differently in space. When the two groups on each carbon atom are on the same side of the double bond, the isomer is called a cis isomer, and when they are on opposite sides, the isomer is called a trans isomer.

For example, consider the following two isomers of 2-butene:

In the cis isomer, the two methyl groups are on the same side of the double bond, while in the trans isomer, they are on opposite sides. The cis/trans isomerism can also occur in cyclic alkenes. For example, consider the following two isomers of cyclohexene:

Summary

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. Due to the restricted rotation about the carbon-carbon double bond, alkenes can exhibit geometric isomerism, which leads to the formation of different isomers. The most common type of geometric isomerism in alkenes is the cis/trans isomerism, which arises when the substituents on each carbon atom of the double bond are arranged differently in space.

Test Your Knowledge:

What is the difference between a cis and a trans isomer in an alkene?

Can geometric isomerism occur in alkenes with more than one carbon-carbon double bond?