Question

If there is no rotation of plane polarised light by a compound in a specific solvent, though to be chiral, it may mean that

• A. the compound is certainly a chiral
• B. the compound is certainly meso
• C. there is no compound in the solvent
• D. the compound may be a racemic mixture

Answer 1

The Correct Option is D

In this question, we need to determine why a compound that is considered chiral does not rotate plane-polarized light when dissolved in a specific solvent. The given options are:

• The compound is certainly achiral.
• The compound is certainly meso.
• There is no compound in the solvent.
• The compound may be a racemic mixture.

Let's evaluate each option step-by-step:

1. The compound is certainly achiral: A compound that is achiral will not rotate plane-polarized light. However, the question specifies that the compound is thought to be chiral. Thus, this option contradicts the premise and can be ruled out.
2. The compound is certainly meso: Meso compounds are a specific class of achiral compounds despite having chiral centers. They have an internal plane of symmetry, making them optically inactive. However, the question indicates that the compound is "thought to be chiral" and not specifically meso. Thus, while possible, this option is not definitive based on the given information.
3. There is no compound in the solvent: If there was no compound in the solvent, indeed, there would be no rotation of plane-polarized light. Though this is a plausible scenario, it does not directly explain the chirality aspect described in the question, and it's more of an assumption about experiment setup rather than chemical property.
4. The compound may be a racemic mixture: A racemic mixture contains equal amounts of two enantiomers, which are non-superimposable mirror images of each other. These enantiomers rotate plane-polarized light in opposite directions by equal amounts, resulting in no net rotation. Hence, this option satisfactorily explains why a compound thought to be chiral shows no optical activity in a solution.

In conclusion, the most logical explanation is that the compound may be a racemic mixture. This accounts for both the perceived chirality of the compound and the lack of optical activity in the solution. Therefore, the correct answer is:

The compound may be a racemic mixture.