Question

Alkenes and cycloalkanes are structural isomers of each other. Give an example of each using ${\mathbf{\text{C}}}_{\mathbf{4}}{\mathbf{\text{H}}}_{\mathbf{8}}$. Another common feature of alkenes and cycloalkanes is that both have restricted rotation about one or more bonds in the compound, so both can exhibit cis-trans isomerism. What is required for an alkene or cycloalkane to exhibit cis-trans isomerism?

Short answer

The examples of${\text{C}}_4{\text{H}}_8$ isomers are But-2-ene andcyclobutane.

REQUIREMENTS:

For an alkene to exhibit cis-trans isomerism, it is important to have different substituents on the two carbon atoms of the double bond. So, the doubly bound carbon atom must have a restricted spin and two non-identical groups.

For cycloalkane to show isomerism, it is important to have two substituents (similar or different) other than hydrogen on the two carbon atoms attached together.

Step-by-step solution

Definition of cis-trans isomerism.

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a chemical word that refers to how atoms are arranged inside molecules.

The requirement for an alkene or cycloalkane to exhibit cis-trans isomerism.

The molecular formula of structural isomers is the same, but the connection is different.

The structural isomers alkenes and cycloalkanes of ${\text{C}}_4{\text{H}}_8$are shown below.

But--ene is an alkene.

Cyclobutane is a cycloalkane.

• But-2-ene shows two structural isomers as shown below.
• Cis-trans isomerism occurs when a molecule's rotation is constrained and each doubly bound carbon atom has two non-identical groups like a hydrogen atom and a methyl group on one carbon as shown below.

• But in cyclobutane, there are no different substituents on any two carbon atoms and therefore, it does not show cis-trans isomerism.
• Here, it is important to have two substituents (similar or different) other than hydrogen on the two carbon atoms attached together.
• The two substituents can be both above the plane (CIS) or one above and one below the plane (TRANS) to show cis-trans isomerism.