Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

An organic molecule necessarily shows optical activity if it (a) is non-superimposable on its mirror image (b) is superimposable on its mirror image (c) contains asymmetric carbon atoms (d) is non-polar

Step-by-Step Solution

Answer

The correct option is (a) non-superimposable on its mirror image.

Step 1: Understand Optical Activity

Optical activity refers to the ability of a molecule to rotate the plane of polarized light. This property is shown by chiral substances which usually contain one or more chiral centers.

Step 2: Define Chiral Centers and Chirality

A chiral center, often a carbon atom, is bonded to four different groups, resulting in non-superimposable mirror images known as enantiomers. The presence of a chiral center typically indicates chirality in the molecule.

Step 3: Analyze each Option

(a) A molecule non-superimposable on its mirror image is chiral and exhibits optical activity. (b) A superimposable molecule is achiral and does not exhibit optical activity. (c) Asymmetric carbon atoms (chiral centers) usually lead to optical activity, but need confirmation. (d) Non-polarity does not necessarily relate to optical activity, as both polar and non-polar compounds can be chiral.

Step 4: Choose the Correct Option

Given the analysis from the previous step, option (a) is correct because non-superimposability directly indicates chirality and optical activity.

Key Concepts

Chiral Centers

A chiral center is a key concept in understanding optical activity. Imagine a central carbon atom bonded to four distinct groups — this is your chiral center. Because these four groups are all different, the spatial arrangement creates two unique, mirror-image structures. Think of your left and right hands; they are mirror images but cannot be perfectly aligned on top of each other. This non-superimposability is the hallmark of chirality in molecules.

• Chiral centers create asymmetry in molecules, crucial for optical activity.
• This property of being different from its mirror image is what makes a molecule chiral.

Recognizing chiral centers in molecules helps in determining whether a substance exhibits optical activity.

Chirality

Chirality is all about asymmetry. If a molecule can exist in two non-superimposable forms due to a chiral center, it is considered chiral. This concept is foundational in organic chemistry. The molecule must have a unique three-dimensional structure that isn’t identical to its mirror image.

• Chirality results in molecules having optical isomers called enantiomers.
• Optically active molecules rotate plane-polarized light.

The concept can be observed in various natural substances, including sugars and proteins where chirality plays crucial roles in their biological functions.

Enantiomers

Enantiomers are pairs of molecules that are mirror images of each other. Despite their similarity in physical properties such as melting points and solubilities, they differ in their interaction with polarized light and biological systems. Enantiomers rotate plane-polarized light in opposite directions – one clockwise (dextrorotatory) and the other counterclockwise (levorotatory).

• Both enantiomers have identical physical properties in achiral environments.
• They often have different effects in biological tissues due to chiral receptors.

Recognizing enantiomers and understanding their behavior is crucial in fields like pharmacology, where one enantiomer may be therapeutic and the other toxic.

Asymmetric Carbon Atoms

Asymmetric carbon atoms, or stereocenters, are central to determining chirality in organic molecules. These atoms are carbon atoms bonded to four unique groups. This asymmetry results in two non-superimposable mirror images or enantiomers. However, the presence of one or more asymmetric carbons doesn't always guarantee optical activity.

• The entire molecular arrangement must be evaluated to affirm optical activity.
• In some cases, internal plane symmetries can nullify chirality despite asymmetric carbons.

Learning how to identify asymmetric carbons is essential for predicting optical activity and for understanding molecular structures labeled as chiral.