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

Short answer

The molecule is optically active if it is non-superimposable on its mirror image (option c).

Step-by-step solution

Understanding Optical Activity

Optical activity refers to a molecule's ability to rotate the plane of polarized light. This property is exhibited by chiral molecules, which are not superimposable on their mirror images.

Chirality and Asymmetry

For a molecule to be optically active, it must be chiral. This generally involves having an asymmetric carbon atom, also known as a stereocenter. Such carbon atoms are bound to four different groups.

Non-Planarity

While non-planarity is a characteristic that some chiral molecules may have, chirality specifically involves molecular symmetry rather than planarity or non-planarity.

Analyzing Superimposability

A key aspect of chirality is non-superimposability on a molecule's mirror image. If a molecule is non-superimposable on its mirror image, it is considered chiral and thus optically active.

Identifying the Correct Option

Given the choices, option (c), 'Is non-superimposable on its mirror image,' directly aligns with the definition of chirality and optical activity. Therefore, this is the correct attribute for optical activity.

Key concepts

Chirality

Chirality is a fascinating concept that describes the geometry of a molecule. Imagine your left and right hands; they are mirror images of each other, but you cannot place your right hand on your left and have all fingers align perfectly. This phenomenon is what scientists refer to as being "chiral." A chiral molecule behaves similarly. It has a non-superimposable mirror image, like your hands.

In the world of chemistry, chirality is crucial because it can significantly affect how molecules interact in biological systems. For instance, the chirality of a molecule can determine the efficacy of a drug. A chiral drug might have completely different effects depending on whether we're looking at it or its mirror image.

• Chirality is about spatial arrangement, not just composition.
• Chiral molecules have an identical structure in terms of constituents but differ in the 3D orientation.

Exploring chirality allows us to predict optical activity, as chiral molecules can rotate the plane of polarized light, making them special in studying molecular interactions.

Asymmetric Carbon Atoms

Asymmetric carbon atoms, often called stereocenters, are at the heart of many chiral molecules. To make a carbon atom asymmetric, it must be bonded to four distinct groups. This unique set-up prevents the molecule from being superimposable on its mirror image, given the varied spatial orientations.

Asymmetric carbon atoms are critical in determining the optical activity of compounds. These atoms serve as the pivot point around which the molecule's chiral properties are expressed.

• Each unique arrangement around an asymmetric carbon introduces the possibility of different stereoisomers.
• This diversity is one reason why specific isomers have different physical and chemical properties, such as optical activity.

By recognizing the role of asymmetric carbon atoms in molecules, we can understand why certain substances behave uniquely under polarized light.

Non-Superimposability

Non-superimposability is a key characteristic of chiral molecules. It refers to the inability to align all parts of a molecule perfectly with its mirror image using rotation or translation. When a molecule and its mirror image cannot be overlaid onto one another, this signifies that the molecule is chiral.

The presence of non-superimposability confirms chirality, making it pivotal for optical activity. Simply put, if you can't stack a molecule directly onto its mirror image, it reveals an inherent three-dimensional asymmetry. This asymmetry is responsible for the molecule's ability to interact with polarized light.

• Non-superimposability means the molecule cannot be rearranged or rotated in a way that matches its mirror counterpart perfectly.
• This feature leads to distinct interactions in biological environments, as one form might be more active or bind differently than the other.

Understanding non-superimposability helps elucidate why chiral molecules are central to both chemistry and biology, especially in applications like pharmaceuticals and molecular biology.