Problem 31 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following compounds will exhibit geometrical isomerism ? [2000S] (a) 1-Phenyl-2-butene (b) 3-Phenyl-1-butene (c) 2-Phenyl-1-butene (d) 1,1-Diphenyl-1-propene

Step-by-Step Solution

Answer

(a) 1-Phenyl-2-butene exhibits geometrical isomerism.

Step 1: Identify the Requirement for Geometrical Isomerism

Geometrical isomerism occurs when a compound has restricted rotation around a double bond and different groups attached to the carbons of the double bond. Typically, these groups must be different for each carbon involved in the double bond.

Step 2: Analyze Each Compound Structure

Let's consider the compounds given: - (a) 1-Phenyl-2-butene: This compound has a structure where the double bond is positioned between the first and second carbon atoms, with a phenyl group on the first carbon. - (b) 3-Phenyl-1-butene: The double bond is located between the first and second carbon atoms, but there is no phenyl group directly attached to the carbons of the double bond to cause variation. - (c) 2-Phenyl-1-butene: Similar to (b), the double bond is between the first and second carbon atoms, but there is a phenyl group on the carbon adjacent to the double bond. - (d) 1,1-Diphenyl-1-propene: The double bond doesn't have different groups on each carbon of the double bond as both possess phenyl groups; thus, symmetry is present.

Step 3: Determine Which Compounds Exhibit Restricted Rotation and Different Groups

Among the compounds, check for differing groups with restricted rotation: - (a) 1-Phenyl-2-butene: The double bond is between carbons 2 and 3, with different groups possible across the double bond. - Both (b) and (c) lack varying substituent groups directly attached to the carbons of the double bond. - (d) has symmetrical groups on each carbon of the double bond, which does not allow for geometric isomerism.

Step 4: Conclude Which Compound Exhibits Geometric Isomerism

After reviewing, only (a) 1-Phenyl-2-butene has the ability to exhibit geometric isomerism due to having different substituents across the double bond, allowing **cis** and **trans** forms.

Key Concepts

Organic Chemistry

Organic chemistry is a branch of chemistry focused on the study of carbon-containing compounds. Carbon's ability to form four covalent bonds makes it remarkably versatile, allowing for the construction of complex molecules, which is why organic chemistry often involves studying the structures and reactions of these compounds.
In the context of geometrical isomerism, organic compounds can showcase a variety of isomers due to the different arrangements possible around their atoms, especially carbons connected by double bonds.

• Isomerism refers to molecules with the same molecular formula but different arrangements of atoms.
• Geometrical isomerism specifically involves different spatial arrangements related to restricted rotation in molecules.

Organic compounds, due to their complex structures, often exhibit different forms of isomerism, including the cis and trans variations, when certain conditions are met.

Double Bond

A double bond consists of two pairs of electrons shared between two atoms - typically carbon atoms - and is often denoted in chemical structures by two parallel lines (=) between the bonded atoms. This bond forms when atoms share two pairs of electrons and provides a level of rigidity to molecular structures because it prevents rotational movement around the bond axis.
The rigidity provided by double bonds is crucial for geometrical isomerism. Here's why:

• Restricted Rotation: Unlike single bonds, double bonds do not freely rotate, hence the spatial arrangement of atoms around the bond is locked.
• Requirement for Isomerism: For geometrical isomerism to occur, each carbon involved in the double bond must have different groups attached.

Thus, whether a given organic compound can display cis or trans isomerism depends significantly on the presence and arrangement around its double bonds.

Cis-Trans Isomerism

Cis-trans isomerism is a common type of geometrical isomerism found in organic chemistry. It arises when two different groups are attached to each carbon of the double bond. The spatial orientation of these groups around the double bond leads to the different isomers:

• Cis Isomer: When two similar or identical groups attached to each carbon of the double bond are on the same side, it's termed a cis isomer.
• Trans Isomer: When these groups are on opposite sides, the compound is known as a trans isomer.

This form of isomerism is pivotal when studying molecular properties because small changes in spatial arrangement (as seen in cis-trans isomers) can significantly alter the physical and chemical properties of a compound.
For example, in 1-Phenyl-2-butene, which exhibits geometrical isomerism, the phenyl group, and other substituents, can exhibit cis or trans configurations resulting in different compound properties. Understanding cis-trans isomerism is essential for predicting and explaining molecular behavior in organic chemistry.