Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Diastereomers are achiral molecules have superposable mirror images.
Reason (R) : Diastereomers are compounds have different physical properties.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Enantiomers have identical physical properties (except the direction they rotate plane-polarized light) and must be separated by chiral methods. Diastereomers have different physical properties and can be separated by standard techniques like fractional distillation or crystallization.

Answer 1

The Correct Option is D

Step 1: Concept:
The question asks to evaluate two statements about stereoisomers, specifically diastereomers, focusing on their chirality and physical properties.

Step 2: Key Definitions:
- Enantiomers: Stereoisomers that are non-superposable mirror images of each other. They must be chiral.
- Diastereomers: Stereoisomers that are NOT mirror images of each other.
- Chirality: A molecule is chiral if it cannot be superposed on its mirror image. Achiral molecules have superposable mirror images.

Step 3: Step-by-step Explanation:

• Evaluating Assertion (A): The statement boldly claims that "Diastereomers are achiral molecules...". This is factually incorrect. While some specific diastereomers (namely, meso compounds) are achiral, the vast majority of diastereomers are chiral. For example, D-glucose and D-galactose are diastereomers, and both are highly chiral molecules. Thus, diastereomers are not defined by being achiral. Assertion (A) is false.

• Evaluating Reason (R): Because diastereomers are not mirror images, they possess different internal distances between atoms and functional groups, resulting in differing overall spatial geometries. As a result, they experience different intermolecular forces. Consequently, diastereomers always have different physical properties, such as boiling points, melting points, densities, and solubilities. Therefore, Reason (R) is a universally true statement.

Step 4: Final Answer:
Since (A) is false and (R) is true, the correct answer is option (D).