Question

Draw and label the cis- and trans-isomers of the molecule represented by the following condensed formula. $${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$$

Short answer

The cis-isomer of the given condensed formula $${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$$ has the two hydrogens on the left carbon of the double bond on the same side as the hydrogen and ethyl group on the right carbon of the double bond: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}90\text{(}=\text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$ The trans-isomer has the two hydrogens on the left carbon of the double bond on the opposite side to the hydrogen and ethyl group on the right carbon of the double bond: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}180\text{(}\equiv \text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$

Step-by-step solution

1. Identify the molecular structure

Based on the given condensed formula $${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$$, the molecular structure can be drawn as: $${\mathrm{H}}_2\mathrm{C}-\mathrm{CH}=\mathrm{CH}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$ Here, the alkene has five carbon atoms with a double bond between the second and third carbon atoms.

2. Create cis-isomer

For the cis-isomer, the two substituent groups ($\mathrm{H}$ and ${\mathrm{CH}}_3$) on the double-bonded carbon atoms are on the same side of the molecule as shown below: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}90\text{(}=\text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$ Where the two hydrogens on the left carbon of the double bond and the hydrogen and ethyl group on the right carbon of the double bond are on the same side.

3. Create trans-isomer

For the trans-isomer, the two substituent groups ($\mathrm{H}$ and ${\mathrm{CH}}_3$) on the double-bonded carbon atoms are on opposite sides of the molecule as shown below: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}180\text{(}\equiv \text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$ Where the two hydrogens on the left carbon of the double bond are on the opposite side to the hydrogen and ethyl group on the right carbon of the double bond.

4. Label the isomers

Now that we have drawn both the cis- and trans-isomers, we can label them accordingly: Cis-isomer: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}90\text{(}=\text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$ Trans-isomer: $${\mathrm{H}}_2\mathrm{C}-\mathrm{C}\text{raisebox}-4pt\text{rotatebox}180\text{(}\equiv \text{)}\mathrm{C}-{\mathrm{CH}}_2{\mathrm{CH}}_3$$

Key concepts

Molecular Structure

Understanding the molecular structure is essential when learning about chemical compounds. A molecule's structure provides information about the arrangement of atoms and bonds, depicting how these are interconnected.
In organic chemistry, a condensed formula such as ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$ can be expanded to visualize its linear structure. For this formula, the molecule consists of a chain of five carbon atoms, with a double bond between the second and third carbons. This double bond is crucial because it determines the potential for cis-trans isomerism.
By visualizing this expanded structure, you can better understand the spatial orientation and potential interactions within the molecule. This foundational knowledge is particularly important when distinguishing between different isomers.

Alkene Chemistry

Alkene chemistry focuses on the study of hydrocarbons containing carbon-carbon double bonds. These double bonds are characterized by the presence of a sigma bond and a pi bond. The restricted rotation around the double bond is a significant factor, which influences the geometry of the molecule.
The existence of a double bond in alkenes, such as in the molecule ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$, allows for the possibility of cis-trans isomerism. This means that the substituents on the carbons involved in the double bond can either be on the same side (cis) or on opposite sides (trans).
Role of the double bond in determining the isomers:

• The position and types of substituents attached to the double-bonded carbons dictate the formation of different geometric isomers.
• This is linked to the difference in properties like melting and boiling points among the isomers.
• Alkene chemistry is greatly enriched by the study of these isomers, which have unique reactivities and physical characteristics.

Structural Isomers

The concept of structural isomerism involves compounds with the same molecular formula but different in the connectivity of their atoms. For alkenes, this type of isomerism is referred to as geometric isomerism, encompassing both cis and trans forms.
In the given exercise, the molecule ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CHCH}}_2{\mathrm{CH}}_3$ can form geometric isomers because of the arrangement around the double bond.
Geometric isomers in focus:

• Cis-isomer: The substituents (such as hydrogen and methyl groups) are on the same side of the double bond.
• Trans-isomer: The substituents are on opposite sides of the double bond.
• These differences arise due to the rigidity of the double bond, leading to distinctly different spatial arrangements.

Key points about structural isomers:

• They have different physical and chemical properties.
• Their unique configurations impact how the molecules interact and react with other chemical species.
• Understanding structural isomers aids in developing more efficient chemical synthesis and industrial applications.