Question

The (R)- and (S)-enantiomers of an optically active compound differ in (CBSE 2000) (1) their reactivity with achiral reagents. (2) their optical rotation of plane-polarized light. (3) their melting points. (4) their solubility in achiral reagents.

Short answer

Option (2) - their optical rotation of plane-polarized light.

Step-by-step solution

Understand enantiomers

Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. They have identical physical properties except for their interaction with plane-polarized light and chiral environments.

Optical activity

Enantiomers rotate plane-polarized light but in opposite directions. One enantiomer rotates it to the right (dextrorotatory) while the other rotates it to the left (levorotatory). This property is known as optical activity.

Reactivity with achiral reagents

Enantiomers generally do not differ in their reactivity with achiral reagents. Therefore, option (1) is incorrect.

Melting points and solubility

Enantiomers have identical melting points and solubility in achiral reagents. This means options (3) and (4) are incorrect.

Correct answer identification

The correct property that distinguishes (R)- and (S)-enantiomers is their optical rotation of plane-polarized light. Therefore, the correct answer is option (2).

Key concepts

Enantiomers

Enantiomers are a fascinating topic in stereochemistry. Think of them like your hands: they are mirror images of each other but cannot be perfectly aligned when placed on top of each other. This is called being 'non-superimposable.' Enantiomers always come in pairs, called R and S forms. These forms are determined by the spatial arrangement of atoms around a chiral center, which is usually a carbon atom bonded to four different groups.
Remember, enantiomers have the same molecular formula and structure, but their orientation in space is different. because of this, they exhibit different behavior in chiral environments, even though their physical properties like melting point and solubility in achiral (non-chiral) reagents remain identical.

Optical Activity

One of the most significant properties of enantiomers is their optical activity. Optical activity refers to the ability of a compound to rotate the plane of plane-polarized light. When plane-polarized light passes through a sample of an optically active compound, it can either rotate to the right or left.
Here's how it works:
- Enantiomers that rotate light to the right (clockwise) are called dextrorotatory and are labeled with a (+) sign.
- Enantiomers that rotate light to the left (counterclockwise) are called levorotatory and are labeled with a (-) sign.
It's important to remember that while enantiomers rotate light in opposite directions, the magnitude of the rotation is the same. This unique interaction with plane-polarized light helps distinguish between the (R) and (S) forms of enantiomers.

Plane-Polarized Light

To understand optical activity, we first need to know what plane-polarized light is. Normal light consists of waves oscillating in all directions perpendicular to its path. However, when light passes through a polarizer, it becomes plane-polarized, meaning the waves all oscillate in a single plane.
This plane-polarized light is crucial in studying chiral molecules and their optical activity because enantiomers will interact differently with this light. A polarimeter is an instrument used to measure the degree of rotation caused by an optically active compound. Based on the direction and angle of rotation, scientists can determine whether a sample is dextrorotatory or levorotatory.
Understanding how plane-polarized light interacts with chiral molecules can provide deep insights into the molecular structures and help in identifying and differentiating enantiomers.