Problem 17 - Chemistry in 30 days

Question

Meso tartaric acid and \(d\)-tartaric acid are: (a) position isomers (b) racemic mixture (c) enantiomers (d) diastereomers

Step-by-Step Solution

Answer

The compounds are diastereomers (d).

Step 1: Understanding the Compounds

Meso tartaric acid and d-tartaric acid are stereoisomers of tartaric acid. Meso tartaric acid is optically inactive because it has an internal plane of symmetry, while d-tartaric acid is optically active, rotating plane-polarized light in one direction.

Step 2: Identifying the Relationship

To determine the relationship between meso tartaric acid and d-tartaric acid, observe their optical activity and symmetry. Enantiomers are non-superimposable mirror images that differ in optical activity, whereas diastereomers are stereoisomers that are not mirror images of each other.

Step 3: Using Optical Activity

Meso tartaric acid does not exhibit optical activity due to its symmetry; it is achiral. In contrast, d-tartaric acid is chiral and optically active. Therefore, they cannot be enantiomers as they do not exhibit opposite optical activity.

Step 4: Understanding Diastereomers

Diastereomers include pairs of stereoisomers that are not mirror images of each other. Meso tartaric acid and d-tartaric acid are diastereomers because they have the same molecular formula but differ in spatial arrangement and optical activity.

Key Concepts

Optical Activity

Optical activity is a fascinating property in chemistry that describes how a compound interacts with plane-polarized light. When a light beam passes through a chiral substance, it can be rotated to the right (dextrorotatory) or to the left (levorotatory). This rotating effect is optical activity.

• Chiral compounds: These are the ones that show optical activity. They don't have an internal plane of symmetry.
• Achiral compounds: These don't show optical activity because of their symmetrical structure, like meso compounds.

Meso tartaric acid doesn't affect plane-polarized light, making it optically inactive because it includes an internal plane of symmetry. On the other hand, \( d \)-tartaric acid does rotate the plane of light due to its chiral nature, making it optically active.
Understanding optical activity can help us differentiate compounds, especially in stereochemistry, where examining the interaction with light helps in determining whether a compound is chiral or achiral.

Diastereomers

Diastereomers are categories of stereoisomers that are not mirror images. Unlike enantiomers that are non-superimposable mirror images, diastereomers have different configurations at one or more (but not all) stereocenters.

• They can have different chemical and physical properties, like boiling points, melting points, and solubilities.
• They are identified when compounds have more than one chiral center but differ only partially in their configuration.

Meso tartaric acid and \( d \)-tartaric acid are considered diastereomers because, despite having the same molecular formula, their atoms are arranged differently in space. The meso version is symmetrical and achiral, meaning it doesn't rotate plane-polarized light, whereas \( d \)-tartaric acid is chiral and rotates light, showcasing their distinction as diastereomers.

Chirality

Chirality is an essential concept in stereochemistry, describing the property of a molecule that makes it non-superimposable on its mirror image. It often comes into play with carbon atoms bonded to four different groups, creating a chiral center.

• A chiral molecule: Cannot be superimposed on its mirror image.
• An achiral molecule: Can be superimposed on its mirror image, often due to possessing a plane of symmetry.

A common example includes organic molecules having chiral centers, such as \( d \)-tartaric acid, where its spatial configuration makes it chiral. In contrast, meso tartaric acid is achiral due to its internal plane of symmetry, even though it has chiral centers. Understanding chirality helps distinguish between molecules that may look similar but behave differently in chemical reactions.