In which of the following properties the enantiomers are different?

Question

Accepted Answer

Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. They have identical physical properties in an achiral environment, such as melting point, boiling point, density, and refractive index. However, they differ in the way they interact with plane-polarized light, which is the basis for their differing optical activity.

1. **Direction of optical rotation (Option 2):** Enantiomers differ in the direction in which they rotate plane-polarized light. One enantiomer will rotate light in a clockwise direction (dextrorotatory, denoted as "+"), while the other will rotate light in a counterclockwise direction (levorotatory, denoted as "-"). This property is unique to chiral molecules and is the primary distinguishing feature of enantiomers. Therefore, Option 2 is correct because it highlights the fundamental difference between enantiomers.

2. **Refractive index (Option 1):** The refractive index is a measure of how much light is bent, or refracted, when it enters a material. Enantiomers have identical refractive indices because this property is not influenced by the chirality of the molecules. Therefore, Option 1 is incorrect.

3. **Melting point and boiling point (Option 3):** These are physical properties that depend on the intermolecular forces within a substance. Enantiomers have identical melting and boiling points because they have the same molecular structure and intermolecular interactions. Thus, Option 3 is incorrect.

4. **Density (Option 4):** Density is a measure of mass per unit volume and is determined by the molecular weight and the arrangement of molecules in a given space. Since enantiomers have the same molecular formula and structure, they also have the same density. Therefore, Option 4 is incorrect.

In conclusion, the direction of optical rotation is the only property among the options listed that distinguishes enantiomers, making Option 2 the correct answer.