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

Short answer

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

Step-by-step solution

Understanding Optical Activity

Optical activity refers to the ability of a compound to rotate the plane of polarized light. This property is related to the structure of the compound, particularly its three-dimensional arrangement and its ability to exist in enantiomeric forms.

Determining Non-Superimposable Mirror Image

A compound is optically active if it can exist as enantiomers, which are non-superimposable mirror images. This is often due to the presence of chirality, where a molecule has a carbon atom bonded to four different groups, making it asymmetric.

Eliminating Superimposable Mirror Image

If a molecule is superimposable on its mirror image, it is achiral and therefore does not show optical activity. Hence, option (b) 'is superimposable on its mirror image' cannot be correct for optical activity.

Relevance of Asymmetric Carbon

Asymmetric (chiral) carbon atoms are commonly associated with optical activity because they provide the molecule with a lack of symmetry, leading to non-superimposable mirror images. This means while the presence of an asymmetric carbon (option c) often indicates potential for optical activity, it is not an absolute that it will always be optically active.

Non-Polar Property

A molecule being non-polar (option d) is unrelated to its optical activity. Polarity is about the distribution of charge within the molecule and does not determine the ability to rotate light.

Conclusion

The best answer is (a) 'is non-superimposable on its mirror image' because it directly describes the condition necessary for a molecule to exhibit optical activity.

Key concepts

Chirality

Chirality is a fundamental concept in organic chemistry. It describes the geometric property of a molecule that makes it non-superimposable on its mirror image. Imagine your left and right hands, which appear as mirror images but cannot be perfectly aligned no matter how you rotate them.
In chemistry, chiral molecules are those that cannot be superimposed on their mirror images, similar to your hands. Chirality is crucial because it often leads to optical activity, where chiral molecules can rotate the plane of polarized light. This quality is vital in distinguishing one enantiomer from another, which can have distinct biological properties.
The presence of chirality in a compound indicates that it might have significant interactions in chemical reactions and biological processes, where the shape and orientation of molecules play a significant role.

Asymmetric Carbon Atoms

Asymmetric carbon atoms are at the heart of chirality. An asymmetric carbon, also known as a chiral center, is a carbon atom bonded to four different groups. This arrangement disrupts any internal symmetry, thereby leading to the potential for non-superimposable mirror images.
These atoms are critical because they set the stage for chirality. When a molecule includes asymmetric carbon atoms, it typically results in the formation of two or more stereoisomers, such as enantiomers, that have different spatial arrangements.

• These variations lead to optical isomerism, a form of stereoisomerism
• Often, the presence of even one asymmetric carbon atom is enough to make a molecule optically active

However, the mere presence of an asymmetric carbon atom does not guarantee optical activity, as the overall structure of the molecule must also be considered.

Enantiomers

Enantiomers are pairs of molecules that are mirror images of each other but are not superimposable. They are a type of stereoisomer that is significant in the study of optically active compounds.
Enantiomers have identical physical properties such as melting point, boiling point, and solubility but differ in how they interact with polarized light. Specifically, they rotate plane-polarized light in opposite directions:

• One enantiomer will rotate light to the left (counterclockwise), termed levorotatory
• The other enantiomer will rotate light to the right (clockwise), termed dextrorotatory

The ability of different enantiomers to rotate polarized light in opposite directions is one way to measure optical activity.
This property plays a crucial role not only in chemistry but also in pharmacology, as different enantiomers of a drug can have very different biological effects.

Polarized Light

Polarized light is central to the study of optical activity. It refers to light waves that vibrate in a single plane, as opposed to unpolarized light which vibrates in multiple planes.
When polarized light passes through an optically active substance, its plane of vibration is rotated. This rotation is directly linked to the specific arrangement within the molecule and the presence of chirality.
To measure the extent of this rotation, scientists use instruments like polarimeters. Optical rotation is quantified in degrees, and certain factors can influence this measurement, including:

• The concentration of the optically active compound
• The path length through which the light travels
• The wavelength of the polarized light
• The temperature at which the observation is made

Understanding how polarized light interacts with optically active substances helps in determining the molecular structure and provides insight into the stereochemistry of the compound.