Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Isomers which can be interconverted through rotation around a single bond are (a) Conformers (b) Diastereomers (c) Enantiomers (d) Positional isomers

Step-by-Step Solution

Answer

(a) Conformers.

Step 1: Understanding the Concept of Isomers

Isomers are compounds that have the same molecular formula but differ in the arrangement of their atoms. There are various types of isomers, including structural isomers and stereoisomers. Stereoisomers include conformers, diastereomers, and enantiomers.

Step 2: Identifying Conformers

Conformers, or conformational isomers, differ from each other by rotation around a single bond. They represent different spatial arrangements of atoms that can occur by rotations about single (sigma) bonds but do not require breaking any bonds.

Step 3: Explanation of Diastereomers and Enantiomers

Diastereomers are stereoisomers that are not related as mirror images and occur in compounds with two or more chiral centers. Enantiomers are stereoisomers that are non-superimposable mirror images of each other, typically requiring chiral centers.

Step 4: Assessing Positional Isomers

Positional isomers are a type of structural isomer where the position of a functional group or substituent on the main chain is altered. Unlike conformers, positional isomers require the breaking and rearrangement of bonds.

Step 5: Relating to the Question

The question asks for isomers that can interconvert through rotation around a single bond, which corresponds to conformers, since this involves the rotation about single bonds without breaking them.

Key Concepts

Conformers

Conformers, also known as conformational isomers, are fascinating because they allow the same molecule to exist in different shapes purely through the rotation around single (sigma) bonds. Imagine holding a rubber band; the way you twist it doesn't change the band itself but alters its appearance. That's similar to what happens with conformers: molecules keep the same structure and formula, but their 3D shapes look different due to internal rotations.
Conformers play a crucial role when analyzing the flexibility and behavior of molecules, especially in biological systems. Certain conformations can be more energetically favorable, meaning they are more likely to be present. For instance, in the molecule ethane, the staggered conformation is more stable than the eclipsed one due to lower steric hindrance and torsional strain. Understanding these can help in predicting molecules' reactivity and interactions.

Stereoisomers

Stereoisomers are a type of isomer where the molecules have the same structural formula but differ only in the spatial arrangement of atoms. This can lead to contrasting properties and functions in biological and chemical contexts. Under the stereoisomer umbrella, we find both enantiomers and diastereomers.

• Enantiomers are mirror images of each other but cannot be superimposed. Perhaps, you can think of your left and right hands; they mirror each other but don't overlap perfectly when placed on one another. Enantiomers require chiral centers and are crucial in pharmaceuticals because one enantiomer could be beneficial while the other might be harmful.
• Diastereomers, on the other hand, differ not as mirror images but in other spatial configurations. They occur when there are multiple chiral centers, and some of those centers vary in configuration.

Stereoisomers highlight how critical 3D arrangements are, impacting how molecules interact within organisms.

Chiral Centers

Chiral centers are pivotal in stereochemistry and occur where a carbon atom is bonded to four distinct groups. This configuration results in non-superimposable mirror images of the given molecule, leading to the creation of enantiomers.
Chirality is essential in many systems, especially in biology, as it determines how a molecule will interact with chiral environments, such as receptors in the human body. For example, the simple molecule lactic acid has a chiral center, and its enantiomers can behave very differently in metabolic pathways. Within chemistry, identifying chiral centers helps in predicting molecular behavior and is essential in designing molecules for specific interactions or reactions.
The term 'chiral' derives from the Greek word for hand, symbolizing how right and left hands are similar yet different—a concept mirrored in molecules possessing chiral centers.