Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{R}\) and \(\mathrm{S}\) enantiomers of an optically active compound differs in (a) their optical rotation of plane polarized light (b) their reactivity with a chiral reagents (c) their solubility in achiral reagents (d) their melting points

Step-by-Step Solution

Answer

R and S enantiomers differ in (a) and (b).

Step 1: Definition and Overview

Enantiomers are chiral molecules that are mirror images of each other. They have the same physical properties except for how they rotate plane-polarized light (

Step 2: Analyze Option (a)

(a) Enantiomers rotate plane-polarized light in equal amounts but in opposite directions. Therefore, R and S enantiomers differ in their optical rotation.

Step 3: Analyze Option (b)

(b) Enantiomers can display different reactivities towards other chiral molecules or reagents due to their different spatial arrangements, so this option also describes a difference between R and S enantiomers.

Step 4: Analyze Option (c)

(c) Since enantiomers have identical physical properties in an achiral environment, they have the same solubility in achiral solvents, making this option incorrect.

Step 5: Analyze Option (d)

(d) Enantiomers have the same melting points in achiral conditions, so R and S enantiomers do not differ in their melting points under these circumstances.

Key Concepts

Optical Rotation

Optical rotation is a fascinating property linked to chiral molecules. When plane-polarized light passes through a solution containing a chiral molecule, the direction of light's vibration is rotated. This phenomenon occurs because the chiral structure of the molecule interacts with light differently depending on its orientation.
For a pair of enantiomers (the R and S forms), each will rotate light in opposite directions. For example, if the R-enantiomer rotates light to the right (called dextrorotatory), its mirror image, the S-enantiomer, will rotate light to the same degree to the left (called levorotatory).
It's important to note that while the direction of rotation differs, the magnitude of rotation (measured in degrees) is identical for both enantiomers. This distinct behavior helps in distinguishing between enantiomers when analyzing substances in the lab.

Chiral Reagents

Chiral reagents are substances that themselves have chiral properties, meaning they have a defined three-dimensional structure that lacks symmetry. Such reagents are prime candidates for reacting differently with enantiomers due to the way they fit together at the molecular level.
Imagine chiral reagents as specialized keys that match specific locks, which in this metaphor are the enantiomers. Because each lock (enantiomer) has a slightly different shape, the key (chiral reagent) fits differently, potentially affecting the reaction outcome.

• This difference in interaction can lead to varied reaction rates.
• It can also produce different products depending on which enantiomer is involved.

Therefore, the use of chiral reagents is crucial in stereoselective reactions, which are vital in fields like pharmaceuticals, where the activity of the molecules can be enantiomer-specific.

Chiral Molecules

Chiral molecules are molecules that cannot be superimposed on their mirror images, much like left and right hands. The concept of chirality is central to the study of stereochemistry and is crucial in understanding the behavior of compounds in biological systems.
A molecule is chiral when it has an asymmetric carbon atom, known as a chiral center, which is bonded to four different groups. The spatial arrangement around this atom makes it possible to have two different molecule configurations that are non-superimposable mirror images of each other.
These configurations are what we call enantiomers. Each enantiomer will have identical physical properties in an achiral environment, like identical melting and boiling points, and solubility, but will behave differently in chiral environments.