Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

Which will exhibit geometrical isomerism here? (a) \(\mathrm{A}_{2} \mathrm{C}=\mathrm{CAB}\) (b) \(\mathrm{A}_{2} \mathrm{C}=\mathrm{CB}_{2}\) (c) \(\mathrm{ABC}=\mathrm{CAB}\) (d) \(\mathrm{ABC}=\mathrm{CX}_{2}\)

Step-by-Step Solution

Answer

Options (a), (b), and (c) can exhibit geometrical isomerism.

Step 1: Understanding Geometrical Isomerism

Geometrical isomerism occurs in compounds with restricted rotation around a bond, typically double bonds (like C=C), where different groups can be positioned differently in space on either side of the bond. The presence of different groups attached to the carbon atoms in the double bond is essential.

Step 2: Analyze Option (a)

For the compound \( \mathrm{A}_{2} \mathrm{C} = \mathrm{CAB} \), consider the double bond \( \mathrm{C} = \mathrm{C} \). The structure has two of the same groups \( \mathrm{A} \) on one carbon and two different groups \( \mathrm{A} \) and \( \mathrm{B} \) on the other, thus this compound can exhibit geometrical isomerism, showing cis and trans forms.

Step 3: Analyze Option (b)

In \( \mathrm{A}_{2} \mathrm{C} = \mathrm{CB}_{2} \), similarly, we have two \( \mathrm{A} \) groups on one side of the double bond and two \( \mathrm{B} \) groups on the other side. This arrangement can lead to geometrical isomerism as either the \( \mathrm{A} \)s or \( \mathrm{B} \)s can be on the same side or opposite sides.

Step 4: Analyze Option (c)

For \( \mathrm{ABC} = \mathrm{CAB} \), each side of the double bond has at least two different groups, allowing for different spatial arrangements. Hence, this compound can potentially exhibit geometrical isomerism.

Step 5: Analyze Option (d)

In \( \mathrm{ABC} = \mathrm{CX}_{2} \), the double bond has one side with different groups (\( \mathrm{A}, \mathrm{B}, \mathrm{C} \)) and the other side with identical groups (\( X \)), so there is no possibility of different spatial arrangements of different groups. Therefore, this compound cannot exhibit geometrical isomerism.

Key Concepts

Cis-Trans Isomerism

Cis-Trans Isomerism, also known as geometrical isomerism, is a form of stereoisomerism. It occurs in compounds with double bonds, where the atoms or groups attached to the carbon atoms can take different positions in space. The term "cis" means "on the same side" and "trans" means "across" or "on opposite sides."

These isomers have the same chemical formula but differ in the positioning of atoms or groups around the double bond. Cis isomers have similar groups on the same side of the double bond, while trans isomers have them on opposite sides.

This isomerism results in different physical and chemical properties even though the structures might seem similar at first glance. For example, cis isomers generally have higher boiling points due to the presence of dipole-dipole interactions, compared to their trans counterparts which are often non-polar and thus have lower boiling points.

In summary, the key to understanding cis-trans isomerism is to focus on the spatial arrangement around the double bond and identifying the sides on which similar groups are positioned.

Double Bonds

Double bonds are a key feature in many organic molecules. Represented as \( \mathrm{C=C} \), they consist of one sigma bond and one pi bond formed between two carbon atoms. Double bonds are characterized by restricted rotation around the bond due to the presence of the pi bond. This restriction is crucial for the formation of geometrical isomers.

The rigidity imposed by the double bond allows for the existence of isomers with different spatial arrangements. Unlike single bonds, which can rotate freely, the pi bond in a double bond locks the bonded atoms in place, preventing rotation.

This rigidity is why compounds with double bonds can exhibit cis-trans isomerism. By preventing rotation, the double bond maintains the distinct positions of the various groups attached, which can lead to either the similar groups being on the same side or opposite sides, creating the conditions necessary for cis-trans isomerism.

Organic Chemistry

In Organic Chemistry, understanding geometrical isomerism forms the foundation for more advanced concepts. Organic chemistry is the study of carbon-containing compounds and their properties, reactions, and structures.

Geometrical isomerism is just one type of isomerism you will encounter in organic chemistry. Isomers are compounds with the same molecular formula but different arrangements of atoms in space, leading to different chemical and physical properties.

Learning to recognize the conditions under which geometrical isomerism occurs—such as the presence of a double bond and different groups attached to the carbons—is an essential skill in organic chemistry.

Understanding these fundamental principles not only aids in predicting the behavior and reactivity of organic molecules but also helps in synthesizing new compounds with desired properties. This foundational knowledge lays the groundwork for more advanced topics like reaction mechanisms and stereochemistry.