Which one of the following statements is true with respect to optical activity?
A. A relationship exists between the (R) and (S) configurations of enantiomers and the direction [(+) or (-)] in which they rotate plane-polarized light.
B. Equimolar mixture of D and L isomers of the same compound will be optically active.
C. Meso compounds that are optically inactive may contain stereogenic centers or chiral centers.
D. Optical activity is independent of temperature.

 

 

 

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What is Optical Activity in Chemistry?

Optical activity refers to a substance’s ability to rotate the plane of polarized light. This is a hallmark property of chiral molecules, but there are common misconceptions about what influences this behavior.

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✅ Correct Statement: C. Meso compounds that are optically inactive may contain stereogenic centers or chiral centers.

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💡 Explanation

Meso compounds contain multiple chiral (stereogenic) centers, but due to an internal plane of symmetry, their optical activities cancel each other out. As a result, even though they contain chiral centers, they are optically inactive.

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Debunking the Incorrect Options

A. Relationship between (R)/(S) and (+)/(-) ❌

• The (R) and (S) designations describe absolute configuration, not optical rotation.
• A compound with (R) configuration can be dextrorotatory (+) or levorotatory (-)—there’s no guaranteed correlation.

B. Racemic Mixtures are Optically Inactive ❌

• An equimolar mixture of D and L isomers (enantiomers) is known as a racemic mixture.
• Their optical rotations cancel each other, making the solution optically inactive.

D. Optical Activity Depends on Temperature ❌

• Optical rotation can change with temperature and wavelength (as per specific rotation formula).
• So, optical activity is not entirely independent of temperature.

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📘 Key Takeaways

• Meso compounds can have chiral centers yet still be optically inactive.
• Optical activity is determined by molecular symmetry, not just chirality.
• Understanding this concept is essential in stereochemistry and pharmaceutical chemistry.