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

(c) \(\mathrm{ABC}=\mathrm{CAB}\) shows geometrical isomerism.

Step 1: Understand Geometrical Isomerism

Geometrical isomerism, also known as cis-trans isomerism, occurs when a compound with a double bond has two different groups attached to each of the carbons involved in the double bond. These compounds can exist as isomers where the position of these groups differs relative to the bond, such as both same-side (cis) or opposite-side (trans).

Step 2: Analyze Compound (a) 

For compound (a) \(\mathrm{A}_{2} \mathrm{C}=\mathrm{CAB}\), the groups attached to the double-bonded carbon are A, A, and C. Since there are identical groups (A) on one carbon, this compound cannot show geometrical isomerism.

Step 3: Analyze Compound (b)

Compound (b) \(\mathrm{A}_{2} \mathrm{C}=\mathrm{CB}_{2}\) has A, A, and B groups attached. Similar to (a), there are identical groups (A and B) on the carbons adjacent to the double bond, so it cannot show geometrical isomerism.

Step 4: Analyze Compound (c)

For compound (c) \(\mathrm{ABC}=\mathrm{CAB}\), we have different groups (A, B, and C) on both sides of the double bond, which can be either same-side or opposite-side. This structure is suitable for geometrical isomerism.

Step 5: Analyze Compound (d)

In compound (d) \(\mathrm{ABC}=\mathrm{CX}_{2}\), the groups involved in the double bond on the one side are all the same (X), implying no geometrical isomerism can occur.

Key Concepts

Cis-Trans Isomerism

Cis-trans isomerism is a fascinating concept in chemistry that falls under the broader category of geometrical isomerism. This type of isomerism requires a compound to have a double bond since the pi-bond restricts free rotation. As a result, the spatial orientation around the double bond can lead to different isomers. The isomers are termed "cis" when similar or identical groups are on the same side of the double bond. Conversely, they are "trans" when similar or identical groups are on opposite sides.

• The cis form often leads to stronger molecular interactions due to proximity.
• Trans isomers tend to have lower polarities as the dipoles may cancel out.

Understanding the difference between cis and trans isomers is crucial when considering the chemical properties, such as boiling points and solubilities, which can vary significantly between the two.

Double Bonds in Molecules

Double bonds play a critical role in the structure and properties of organic molecules. They consist of one sigma bond and one pi bond. The pi bond is responsible for the rigidity of the bond, preventing free rotation and thus making geometrical isomerism possible. In simple terms, because the pi bond locks the position of the atoms or groups attached to the carbons, isomers with different configurations can exist.

• Double bonds are shorter and stronger than single bonds.
• This rigidity helps establish certain molecular geometries important in determining the reactivity and interaction of molecules.

When attempting to determine the isomerism potential in a given compound, always check the groups attached to either carbon in the double bond. They must be different to exhibit geometrical isomerism.

Molecular Structure Analysis

Analyzing the molecular structure of compounds to determine geometrical isomerism can seem daunting at first. However, breaking down the steps can make this task easier. Start by identifying the presence of a double bond, which is essential for geometrical isomerism. Next, check the substituent groups attached to the carbons forming the double bond. For a compound to exhibit geometrical isomerism, each carbon involved in the double bond must have two different groups attached.

• Examine each carbon for symmetry.
• Differentiate whether groups can align in a cis or trans configuration.
• Consider how these configurations might affect the compound's properties and reactivity.

Through molecular structure analysis, you gain insight into how the positioning of atoms within a molecule affects its physical and chemical properties.