Question

Which class of compound can exhibit geometrical isomerism (a) \(\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{NOH}\) (d) All of these

Short answer

(a) and (b) can exhibit geometrical isomerism, so not all of these.

Step-by-step solution

Understanding Geometrical Isomerism

Geometrical isomerism occurs in compounds with restricted rotation around a bond. Most often, this is seen in compounds with double bonds, where different groups are attached to the carbon atoms of the double bond. This results in cis and trans configurations, depending on the relative positions of these groups.

Analyzing Compound (a)

The compound (a) CH₃CH=CHCH₃ contains a double bond between two carbon atoms. Both carbon atoms in this double bond have two different substituents: one has a hydrogen and a CH₃ group, and the other also has a hydrogen and a CH₃ group. Hence, this compound can exhibit geometrical isomerism.

Analyzing Compound (b)

The compound (b) C₆H₅CH=NOH also contains a double bond, where the carbon atom is attached to a phenyl group and a hydrogen, and the nitrogen is attached to a hydroxyl group. Here, the restriction in rotation around the double bond allows the possibility of geometrical isomerism, with different spatial orientations of the groups.

Conclusion on Possible Answers

Since both (a) and (b) have the necessary conditions for geometrical isomerism, they both can exhibit this type of isomerism. As option (d) suggests all of these, and since we have only considered options (a) and (b) as viable, option (d) is not correct unless we have a (c) that meets all requirements.

Key concepts

Double Bonds

Double bonds are a key feature in organic chemistry, particularly when discussing geometrical isomerism. They consist of one sigma bond and one pi bond between two adjacent atoms, typically carbon. This double bond creates a scenario where rotation around the bond is restricted, unlike single bonds which allow free rotation. This restriction is significant because it influences the spatial arrangement of the molecules, resulting in different isomers based on how substituents are positioned relative to each other.

In terms of appearance and functionality, the lack of rotation around a double bond is crucial for the formation of distinct molecular shapes, especially in geometry-laden molecules. This fixed positioning can alter the chemical properties and interactions of the molecule. The rigidity of double bonds makes them a typical site for geometric isomerism to occur, which occasionally results in different chemical and physical properties between the isomers. Understanding this concept is essential, as it lays the foundation for recognizing isomerism in more complex molecules.

Cis-Trans Isomerism

Cis-trans isomerism, a subset of geometric isomerism, arises due to the restricted rotation about a double bond. It is most easily recognized in compounds where two substituents are positioned differently about a double bond.

• **Cis Isomer:** The substituents are on the same side of the double bond. This can often result in higher polarity and therefore different physical and chemical properties.
• **Trans Isomer:** The substituents are on opposite sides of the double bond, likely reducing any symmetry-related polarity differences.

The cis-trans naming system originates from Latin, with "cis" meaning "on this side" and "trans" meaning "across." This variation plays an essential role in determining the interaction of the molecule with other molecules, influencing properties like boiling points, solubility, and reactivity. It also has significant implications in biological systems and synthetic chemistry. Cis-trans isomers are vital for students to understand, as they exemplify how minor changes in structure can lead to major changes in a compound's properties.

Organic Chemistry

Organic chemistry, often known as the chemistry of carbon compounds, forms the backbone of many processes and substances. It encompasses the structure, properties, composition, reactions, and preparation of carbon-containing compounds. These compounds typically contain carbon-hydrogen bonds, and can also contain elements like nitrogen, oxygen, sulfur, phosphorus, and various metals.

A fundamental focus of organic chemistry is the study of hydrocarbons—molecules composed entirely of carbon and hydrogen—and their derivatives. Isomerism is an important aspect within this realm, highlighting how the same set of atoms can rearrange to form different structures, leading to compounds with varied properties.

Learning about organic chemistry involves understanding concepts such as:

• Different types of bonds (single, double, triple).
• Various functional groups and their reactivity.
• Stereochemistry, including an appreciation for chirality and isomerism.

By grasping these foundational elements, students can appreciate how organic chemistry is integral to various fields, from pharmaceuticals and biotechnology to petrochemicals and materials science. By understanding complex molecules and reactions, students can apply this knowledge to innovatively solve real-world problems.