Question

Which of the following compounds can exist as pairs of cis-trans isomers? Draw each cis-trans pair, and indicate the geometry of each isomer. (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}\) (b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (e) \(\mathrm{ClCH}=\mathrm{CHCl}\) (f) \(\mathrm{BrCH}=\mathrm{CHCl}\)

Short answer

Compounds (c), (e), and (f) can have cis-trans isomers.

Step-by-step solution

Understand cis-trans isomerism

Cis-trans isomerism (also known as geometric isomerism) occurs in alkenes where there are different groups attached to each carbon of the double bond. A cis isomer has the similar or higher-priority groups on the same side, while a trans isomer has them on opposite sides. For a compound to show cis-trans isomerism, each carbon of the double bond must have different substituents.

Analyze each compound

Let's go through each compound to determine if they can possess cis-trans isomers based on the steric requirements: (a) In \( \mathrm{CH}_{3} \mathrm{CH} = \mathrm{CH}_{2} \), the carbon on the right has two hydrogen atoms, so it cannot form cis-trans isomers. (b) \( (\mathrm{CH}_{3})_{2} \mathrm{C} = \mathrm{CHCH}_{3} \) has the same substituents on one carbon of the double bond, so it cannot form cis-trans isomers. (c) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CHCH}_{3} \) has different groups on each carbon of the double bond, so it can form cis-trans isomers. (d) \( (\mathrm{CH}_{3})_{2}\mathrm{C} = \mathrm{C}(\mathrm{CH}_{3})\mathrm{CH}_{2}\mathrm{CH}_{3} \) has the same substituents on both carbons of the double bond, so it cannot form cis-trans isomers. (e) \( \mathrm{ClCH} = \mathrm{CHCl} \) has different groups on each carbon of the double bond, so it can form cis-trans isomers. (f) \( \mathrm{BrCH} = \mathrm{CHCl} \) has different groups on each carbon of the double bond, so it can form cis-trans isomers.

Draw and label cis-trans isomers

For compounds (c), (e), and (f) that can exhibit cis-trans isomerism, draw and label the isomers: - For \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CHCH}_{3} \): - **Cis**: Both \( \mathrm{CH}_{3} \) groups on the same side. - **Trans**: \( \mathrm{CH}_{3} \) groups on opposite sides.- For \( \mathrm{ClCH} = \mathrm{CHCl} \): - **Cis**: Both \( \mathrm{Cl} \) atoms on the same side. - **Trans**: \( \mathrm{Cl} \) atoms on opposite sides.- For \( \mathrm{BrCH} = \mathrm{CHCl} \): - **Cis**: \( \mathrm{Br} \) and \( \mathrm{Cl} \) on the same side. - **Trans**: \( \mathrm{Br} \) and \( \mathrm{Cl} \) on opposite sides.

Key concepts

Geometric Isomerism

Geometric isomerism, often called cis-trans isomerism, is a fascinating concept within the realm of organic chemistry. It specifically relates to the orientation of atoms within molecules possessing carbon-carbon double bonds, known as alkenes. These double bonds restrict free rotation, causing different spatial arrangements of substituents leading to distinct isomers. In geometric isomers:

• "Cis" refers to isomers where similar groups are on the same side of the double bond.
• "Trans" refers to isomers where similar groups are on opposite sides.

Understanding geometric isomerism is crucial because each isomer can possess different physical and chemical properties, which can significantly impact how substances act in biological and chemical processes.

Alkenes

Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (\( \text{C} = \text{C} \)). These double bonds are characteristic of alkenes and differentiate them from alkanes, which only have single bonds. The double bond is not just a point of attachment, but it's central to the chemistry of alkenes, influencing their reactivity and the concept of geometric isomerism.
Alkenes are notable for the following:

• They are unsaturated hydrocarbons due to the double bond which allows for additional chemical reactions like addition reactions.
• The presence of a double bond influences the spatial geometry and prevents the free rotation of bonded atoms, contributing to the formation of isomers.

Their unique structure makes them pivotal in organic synthesis and industrial applications, including the manufacturing of plastics and alcohols.

Substituents

Substituents are groups or atoms that are attached to the main carbon chain or backbone of organic molecules, significantly affecting the molecule's properties and reactions. In the context of cis-trans isomerism, the nature and arrangement of substituents play a critical role.
Key characteristics of substituents include:

• They determine whether a molecule can exhibit cis-trans isomerism, as different atoms/groups on each carbon of a double bond are necessary.
• Substituents can alter reactivity and interactions with other substances due to differences in size, electronegativity, and polarity.

Thus, understanding substituents is fundamental for predicting molecule behavior, aiding in the design and synthesis of new compounds with desired traits.

Stereochemistry

Stereochemistry is the study of the spatial arrangement of atoms in molecules and its influence on chemical properties and reactions. It is a broad area encompassing various types of isomerism, including geometric (cis-trans) isomerism.
Important aspects of stereochemistry include:

• It helps in understanding how different spatial arrangements of atoms affect molecule function and interaction with other molecules in chemical reactions.
• Stereochemistry is vital in fields like pharmacology, where the shape of a molecule can significantly influence its effectiveness and interaction with biological systems.

Through stereochemistry, chemists can appreciate the 3D nature of molecules, leading to innovations in drug development, materials science, and biotechnology.