Problem 146 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{aligned} &\text { Match the following }\\\ &\begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) Enantiomers } & \text { (p) Equilibrium mixture } \\ \text { (b) Diastereomers } & \text { (q) Mirror images } \\ \text { (c) Meso isomer } & \text { (r) Non- mirror images } \\ \text { (d) Tautomers } & \text { (s) Non- superimposable } \\ & \text { (t) Stereo isomer } \\ \hline \end{array} \end{aligned} $$

Step-by-Step Solution

Answer

(a) - (q), (s); (b) - (r), (t); (c) - None specifically; (d) - (p).

Step 1: Understanding Enantiomers

Enantiomers are non-superimposable mirror images of each other. This means that they are like left and right hands - mirror images, but cannot be placed on top of each other and look the same. From Column-II, enantiomers relate to (q) mirror images and (s) non-superimposable.

Step 2: Understanding Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other and usually have different physical properties. From Column-II, diastereomers match with (r) non-mirror images and (t) stereo isomer.

Step 3: Understanding Meso Isomer

Meso isomers contain multiple stereocenters but also have an internal plane of symmetry that makes them achiral (they are superimposable on their mirror images). Meso isomers typically do not match any direct options listed in Column-II, but they relate to the concept of (s) non-superimposable by being superimposable on their mirror image.

Step 4: Understanding Tautomers

Tautomers are isomers of a compound which readily interconvert by a chemical reaction, usually involving the relocation of a proton. This is often found in dynamic equilibria. From Column-II, tautomeric forms are described by (p) equilibrium mixture.

Key Concepts

Enantiomers

Enantiomers are fascinating in the world of stereochemistry. They are molecules that are mirror images of each other, like your left and right hands. Imagine looking at your reflection in the mirror; everything appears flipped. Enantiomers are similar. They have the same atoms and bonds but are arranged differently in space, making them non-superimposable on one another. Think of them as twins that are not identical twins. They share everything in common except their handedness. This unique nature is vital in fields such as pharmaceuticals, where one enantiomer of a drug might be therapeutic, while its mirror image could be less effective or even harmful. So, when you learn about enantiomers, remember: mirror images but not superimposable. They don't just look different; they behave differently, especially when interacting with other chiral molecules.

Diastereomers

Diastereomers hold a special spot in stereochemistry, distinct from enantiomers. These are also stereoisomers but they are not mirror images of each other. They can differ at one or more stereocenters, meaning they have different spatial arrangements of atoms, leading to varied physical and chemical properties.
For example, a diastereomer might boil or melt at a different temperature compared to another diastereomer of the same compound. In simpler terms:

• They are not mirror images.
• They are not superimposable overall.
• They show differences in physical properties like color, solubility, and melting point.

Understanding diastereomers can be very handy, especially when studying organic reactions and determining why certain reactions lead to multiple products. They are like brothers with different tastes and personalities.

Meso Isomers

Meso isomers present an interesting twist to the concept of chirality. Although they have stereogenic centers, which typically lead to chirality, meso compounds are achiral. This is because they possess an internal plane of symmetry. Picture a perfectly symmetrical object that looks the same if you cut it down the middle and flip one of the halves over. That's a meso compound.
Despite having chiral centers, the overall molecule is symmetrical, thus, it can be superimposed on its mirror image. Key characteristics include:

• They contain multiple stereocenters.
• They appear achiral due to symmetry.
• The internal plane divides it perfectly into mirrored halves.

This makes them important in synthesis, where achieving or avoiding chirality is crucial. Meso compounds are unique because they challenge our understanding of chirality itself.

Tautomers

Tautomers introduce a dynamic aspect to isomerism. Unlike the static nature of structural isomers, tautomers can quickly interconvert, especially through a proton shift. This process is often seen in dynamic equilibria, where two tautomeric forms coexist in a mixture. For instance, think of the keto-enol tautomerism, where the position of a hydrogen and a double bond change, creating equilibrium between the two forms.
The characteristics of tautomers include:

• They are quickly interconvertible.
• Commonly involve the migration of a hydrogen atom.
• Forms participate in equilibrium mixtures.

Their ready transformation means they often play a significant role in biochemical processes, like the base-pairing in DNA replication, and in designing drugs where tautomeric balance dictates efficacy. Understanding tautomers helps chemists predict and control reactions more effectively.