Question

A racemic mixture is a mixture of (1) meso and its isomers. (2) \(d\) - and its \(l\) -isomers in equal proportions. (3) \(d\) - and its \(l\) -isomers in different proportions. (4) meso and \(d\) -isomers.

Short answer

Option (2) is correct: a racemic mixture contains \(d\) and \(l\) isomers in equal proportions.

Step-by-step solution

Understanding Racemic Mixture

A racemic mixture is a mixture containing equal amounts of two enantiomers, which are molecules that are mirror images of each other but not superimposable.

Identify the Correct Option

Among the given options, find the one that describes a mixture of equal amounts of enantiomers.

Evaluate Each Option

Option (1): This describes a mixture of meso and its isomers, which does not fit the definition. Option (3): This describes a mixture of enantiomers in different proportions, which is not a racemic mixture. Option (4): This includes meso and an isomer, which is incorrect for a racemic mixture. Option (2): This matches the definition as it states a mixture of \(d\) and \(l\) isomers in equal proportions.

Conclusion

Based on the evaluation, the correct answer is option (2), which describes a racemic mixture as equal parts of \(d\) and \(l\) isomers.

Key concepts

enantiomers

Enantiomers are a special type of isomer. These molecules are mirror images of each other, like your left and right hands. However, despite having identical structures, they cannot be superimposed on one another. This unique feature is what distinguishes enantiomers from other types of isomers.
The presence of a chiral center, usually a carbon atom attached to four different groups, is essential for enantiomers. Because they are mirror images, they often have different effects in biological systems. For example, one enantiomer of a drug might be therapeutic, while the other could be inactive or even harmful.
When you hear about enantiomers, think of them as a pair of twins that look alike but can never perfectly overlap.

isomers

Isomers are molecules with the same molecular formula but different structures or arrangements. They can be categorized into several types:
- Structural isomers: These have different connections between atoms. For example, butane and isobutane both have the molecular formula C4H10, but their atoms are connected differently.
- Stereoisomers: These have the same connections between atoms but differ in the spatial arrangement of their atoms. Enantiomers fall into this category.
Understanding isomers is important because their differing structures often lead to different chemical and physical properties. This is why the study of isomers is crucial in fields like pharmacology and materials science.

equal proportions

In the context of chemistry, equal proportions mean that two or more components are present in the same amount, or quantity, by either weight or volume. When we talk about a racemic mixture, we refer to an equal mix of two enantiomers. For example, a racemic mixture of the enantiomers denoted as 'd' and 'l' will have 50% 'd' and 50% 'l'. This balance is crucial because it leads to a mixture with no net optical activity. In other words, the ability of one enantiomer to rotate plane-polarized light in one direction is exactly canceled out by the other enantiomer rotating it in the opposite direction.
So, when dealing with racemic mixtures, remember that equal proportions of enantiomers are necessary to achieve a balanced chemical behavior.