Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

A similarly between optical and geometrical isomerism is that (a) if in a compound, one is present then so is the other. (b) each forms equal number of isomers for a given compound (c) both are included in stereoisomerism (d) they have no similarly

Step-by-Step Solution

Answer

Option (c): Both are included in stereoisomerism.

Step 1: Understand the Types of Isomerism

Identify the types of isomerism involved. Geometrical isomerism arises due to different spatial arrangements of groups around a double bond or a ring, leading to cis-trans configurations. Optical isomerism, on the other hand, occurs when a molecule can exist in two forms that are mirror images of each other, usually involving a chiral center.

Step 2: Define Stereoisomerism

Recognize that stereoisomerism is a broader category, which includes isomers that have the same structural formula but differ in the spatial arrangement of atoms. Specifically, both geometrical and optical isomerism fall under the category of stereoisomerism.

Step 3: Evaluate the Options

Review each given option: (a) implies a mutual presence that is incorrect because it's possible for a compound to exhibit only one form; (b) suggests an equal number of isomers, which is incorrect because the number depends on specific molecular details; (c) both types fall under stereoisomerism, which is correct; (d) states there is no similarity, which is incorrect as discussed in Step 2.

Step 4: Select the Correct Option

Based on the analysis, choose option (c) because both optical and geometrical isomerism are types of stereoisomerism, which is their similarity.

Key Concepts

Optical Isomerism

Optical isomerism is a fascinating type of isomerism that occurs in molecules. The key feature of optical isomerism is its focus on chirality. Chirality comes into play when a molecule has a carbon atom with four different groups attached to it. This carbon is called a chiral center. If you imagine holding a molecule in front of a mirror, an optical isomer has its non-superimposable mirror image. If you try to stack these mirror images on top of each other, they don't match perfectly, just like left and right hands.
This unique property of having non-superimposable mirror images leads to two optical isomers known as enantiomers. They have identical physical properties but differ in how they interact with polarized light. One enantiomer will rotate polarized light in one direction, while the other will rotate it in the opposite direction.

• Chiral center: A carbon atom bonded to four different groups.
• Mirror images: Non-superimposable and similar to left and right hands.
• Enantiomers: Two optical isomers that rotate polarized light in opposite directions.

These specific interactions with light make optical isomerism incredibly important in fields like biology and pharmacology.

Geometrical Isomerism

Geometrical isomerism, another captivating type of isomerism, arises mainly in compounds with double bonds or within rings. The lack of rotation around these rigid structures causes the formation of different spatial arrangements known as isomers. Typically, you will encounter terms like 'cis' and 'trans' when discussing geometrical isomers.
In a 'cis' isomer, similar groups are on the same side of the double bond or ring. In contrast, a 'trans' isomer has similar groups on opposite sides. This produces a considerable difference in physical and chemical properties. For instance, melting and boiling points can vary between cis and trans isomers because of the different interactions between molecules.

• Double bonds/rings: Prevent rotation, leading to fixed spatial arrangement.
• Cis isomer: Similar groups on same side.
• Trans isomer: Similar groups on opposite sides.

These unique characteristics make geometrical isomerism significant in chemical reactions and material properties.

Stereoisomerism

Stereoisomerism is a broad category that encompasses isomers whose atoms are connected in the same order but differ in their spatial arrangement. This fascinating type of isomerism is divided into two main types: optical isomerism and geometrical isomerism. This common characteristic of spatial arrangement makes them a crucial topic in the study of chemistry.
All stereoisomers share the same skeletal structure but will exhibit different three-dimensional configurations. Unlike regular structural isomers, which differ completely in how atoms are bonded or connected, stereoisomers distinguish themselves through their unique arrangement in space.

• Includes both optical and geometrical isomerism.
• Same bond connections, different spatial arrangements.
• Significant in activities like understanding drug efficacy and material properties.

This importance highlights stereoisomerism as a fundamental aspect of studying how molecular shape influences both physical properties and chemical behavior.