Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(c) Non-superimposable on its mirror image.

Step 1: Identifying Optical Activity

Optical activity refers to the molecule's ability to rotate the plane of polarized light. To determine which condition guarantees optical activity, we need to explore the relationship between molecular structure and optical behavior.

Step 2: Understanding Key Concepts

A molecule is optically active if it is chiral. Chirality implies that the molecule and its mirror image cannot be superimposed on each other, leading to optical activity when the molecule is placed in a polarized light.

Step 3: Evaluating Each Option

(a) An asymmetric carbon atom, also known as a chiral center, does not always guarantee optical activity because the molecule might have a plane of symmetry or might be part of a racemic mixture. (b) Non-planar molecules are not necessarily chiral and might be superimposable on their mirror image. (c) Non-superimposable mirror images are a direct implication of chirality and therefore guarantee optical activity. (d) Superimposable mirrors images indicate lack of chirality and no optical activity.

Step 4: Selecting the Correct Option

Among the options presented, (c) 'non-superimposable on its mirror image' directly describes a condition for chirality, thus ensuring optical activity. This is the most suitable condition reflecting the fundamental property of chiral molecules.

Key Concepts

Chirality

Chirality is a fascinating concept in chemistry that plays a crucial role in understanding molecular structure. In simple terms, a molecule is chiral if it cannot be superimposed on its mirror image.
This characteristic is often compared to how our left and right hands are mirror images but cannot be perfectly aligned on top of each other.
Here are some essential aspects of chirality:

• Unique Structure: A chiral molecule has a specific three-dimensional arrangement.
• Optical Activity: Chiral molecules interact with polarized light, causing it to rotate. This property is known as optical activity, which is measurable in a lab.
• Enantiomers: These are pairs of molecules that are non-superimposable mirror images of each other.

Chirality is not limited to large, complex molecules; even simple organic compounds can be chiral if they meet the necessary criteria. Understanding this concept is vital in fields like pharmaceuticals, where the different orientations of chiral molecules can lead to significantly different biological effects.

Asymmetric Carbon

An asymmetric carbon, also known as a chiral center, is integral to a molecule’s chirality. It is a carbon atom bonded to four different groups. The presence of one or more asymmetric carbons is often a strong indicator of chirality.
Let’s delve into why:

• Distinct Groups: For a carbon to be asymmetric, each of the four groups attached to it must be different, creating a unique three-dimensional configuration.
• Role in Stereochemistry: Asymmetric carbons are essential in defining the spatial arrangement of atoms in a molecule, which is vital for understanding its reactivity and interaction with other molecules.

It is important to note that while the presence of an asymmetric carbon often leads to chirality, it is possible for a molecule with an asymmetric carbon to be non-chiral if there is another element of symmetry that cancels out its effects. This is why simply having an asymmetric carbon is not a guarantee of chirality or optical activity.

Non-superimposable Mirror Image

A molecule’s non-superimposable mirror image goes hand-in-hand with the concept of chirality. A non-superimposable mirror image refers to a situation where the mirror image of a molecule cannot be aligned with the original molecule by any rotation or translation.
Here’s how this concept is understood:

• Defining Feature of Chirality: Non-superimposable mirror images are the cornerstone of identifying chiral molecules.
• Optical Isomerism: Molecules with non-superimposable mirror images often exhibit optical activity, meaning they can rotate polarized light in different directions.

This characteristic is used in various fields, especially in determining the properties of molecules in organic chemistry. When analyzing a molecule's structure, checking for non-superimposable mirror images helps in predicting how the molecule will behave in chemical reactions. It is a key indicator of the molecule's ability to interact uniquely with other chiral entities, such as enzymes and receptors within biological systems.