Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

All of these compounds (a, b, and c) can exhibit geometrical isomerism.

Step 1: Understand Geometrical Isomerism

Geometrical isomerism occurs in alkenes with restricted rotation around the double bond, requiring different groups attached to the double-bonded carbons for isomer formation.

Step 2: Analyze Compound (a)

Compound (a) \( \text{CH}_3\text{CH}=\text{CHCH}_3 \)can exhibit geometrical isomerism as it has a double bond with the two carbon atoms connected to different groups, which are \(\text{CH}_3\) and \(\text{H}.\)

Step 3: Analyze Compound (b)

In compound (b) \(\text{C}_6\text{H}_5\text{CH}=\text{NOH} \), the carbon-nitrogen double bond can exhibit geometrical isomerism as the bonded groups are different, \(\text{C}_6\text{H}_5 \) and \(\text{H}\) on one side and \(\text{OH}\) and an implied hydrogen on the nitrogen side.

Step 4: Analyze Compound (c)

Compound (c) which can be expressed from its SMILES notation asdrawn compound with structure like \(\text{HOOC-CH=CH-COOH}\), with two different groups on each carbon of the double bond, allowing for cis-trans isomerism.

Step 5: Conclusion on Geometrical Isomerism Possibility

Since all compounds (a), (b), and (c) can exhibit geometrical isomerism, the answer to the question is option (d), 'All of these.'

Key Concepts

Alkenes

Alkenes are a class of hydrocarbons characterized by the presence of a carbon-carbon double bond. This double bond serves as a key defining feature of alkenes, setting them apart from other hydrocarbons like alkanes, which only have single bonds. Understanding alkenes involves recognizing their distinct properties stemming from this functional group.

Some important points to consider about alkenes include:

• They follow the general formula \( ext{C}_n ext{H}_{2n}\), which helps to easily identify them in chemical notation.
• They are unsaturated hydrocarbons, meaning their carbon atoms are not bonded to the maximum number of hydrogen atoms possible, allowing for additional reactions.
• Alkenes often participate in addition reactions due to their double bond, where atoms or groups are added across the bond.

These properties make alkenes versatile and important in chemical synthesis and industrial applications.

Double Bond

The double bond in alkenes is a pivotal structural feature that substantially influences their chemical behavior. These bonds consist of one sigma bond and one pi bond, holding two carbon atoms together. This arrangement creates a region of electron density above and below the plane of the atoms, resulting in a rigid part of the molecule.

Here’s what distinguishes double bonds:

• The pi bond restricts rotation around the double bond, leading to stable configurations that can result in isomers.
• Double bonds have a bond angle of roughly 120 degrees due to the hybridization of the orbiting electrons.
• This bond alters the physical properties of alkenes, such as lowering their boiling points compared to similar alkanes.

Because of the restricted rotation caused by double bonds, alkenes often exhibit the intriguing phenomenon of geometrical isomerism.

Cis-Trans Isomerism

Cis-trans isomerism is a specific type of geometrical isomerism that arises in molecules with restricted rotation around a double bond. It happens when there are distinctly different groups attached to both sides of a double bond, allowing for two different spatial arrangements of the atoms.

For a compound to display cis-trans isomerism, it must meet certain conditions:

• The double-bonded carbons must each bear two different atoms or groups for it to be possible.
• The 'cis' isomer has identical or similar groups on the same side of the double bond.
• The 'trans' isomer has the groups on opposite sides, leading to different physical and chemical properties.

This kind of isomerism plays an important role in fields like chemistry and biology due to the varying properties of cis and trans isomers, such as their different boiling points and reactivity.

Restricted Rotation

Restricted rotation is a fundamental concept that describes the difficulty of rotation around a double bond. This is due to the presence of the pi bond, which locks the region between the two atoms in place. Free rotation is common in single bonds but not in double bonds found in alkenes.

Here’s why restricted rotation matters:

• It ensures that the spatial arrangement of atoms in a molecule remains fixed, a prerequisite for geometrical isomerism like cis-trans isomerism.
• This restriction gives rise to distinct isomers with unique configurations and properties.
• Without restricted rotation, the different spatial arrangements wouldn’t have stable identities or result in the same molecule always being formed regardless of conditions.

The concept of restricted rotation highlights why molecules with double bonds can exhibit vastly different properties just by having different spatial configurations.