Question

Which among the following represents diastereomeric pair?

Short answer

The diastereomeric pair can be identified by analyzing the given pairs of molecules, identifying their stereocenters, assigning their configurations as (R) or (S), comparing the stereocenters, and choosing the pair with at least one stereocenter the same and at least one stereocenter different.

Step-by-step solution

Identify the stereocenters and their configurations

First, look at the given options and identify the stereocenters (atoms with four different groups attached). Also, assign the configurations at each stereocenter (R or S). Step 2 - Compare the stereocenters

Compare the stereocenters in the pairs

Compare the stereocenters between the molecules in each pair and determine whether any pairs have the same connectivity, but at least one stereocenter different, and at least one is the same. Example: Consider a pair of molecules A and B. A: \(\begin{array}{ c c c } C & - & C \\ | & C & | \\ H & - & H \\ \end{array}\) B: \(\begin{array}{ c c c } C & - & H \\ | & C & | \\ H & - & C \\ \end{array}\) In this example, the first stereocenter for A is (R) and B is (S). The second stereocenter for A is (R) and B is (R). Since one stereocenter is the same, and one stereocenter is different, the pair represents diastereomers. Step 3 - Identify the diastereomeric pair

Identify the diastereomeric pair

After comparing the stereocenters for each pair of molecules, the pair that represents diastereomers is the one that fulfills the criteria of having at least one stereocenter the same and at least one stereocenter different. Apply these steps to the given options to identify the diastereomeric pair.

Key concepts

Understanding Stereocenters

The concept of a stereocenter is fundamental in organic chemistry, playing a pivotal role in the structure and behavior of molecules. A stereocenter, also known as a chiral center, refers to an atom that is attached to four different groups, creating an arrangement where the molecule cannot be superimposed on its mirror image. This unique spatial arrangement is what gives rise to isomerism, where compounds have the same molecular formula but differ in the three-dimensional orientations of their atoms.

To visualize a stereocenter, imagine a central carbon atom with four different substituents; each of these substituents can be arranged in space in two different ways, leading to non-superimposable mirror image structures, known as enantiomers. However, not all molecules with stereocenters are chiral; some can have symmetric arrangements that allow for superposition, which is often a point of confusion for students.

Using molecular models or three-dimensional drawings can aid significantly in understanding and identifying stereocenters in complex molecules. Identifying the presence and configuration of stereocenters helps us predict the behavior and interactions of molecules, making it a crucial skill in fields such as drug design and synthesis.

R and S Configuration - Assigning Stereochemistry

When it comes to assigning the R and S configuration to stereocenters, understanding the Cahn-Ingold-Prelog priority rules is essential. These rules help determine the configuration by assigning priorities to the substituents attached to the stereocenter based on atomic number: the higher the atomic number, the higher the priority.

After ranking the substituents, we orient the molecule so that the lowest priority group is away from us, and then trace a path from the highest priority group to the lowest ranked group that is visible. An R (from the Latin rectus, meaning right) configuration results if this path goes in a clockwise direction, while an S (from the Latin sinister, meaning left) configuration is assigned if the path is counterclockwise.

One common challenge students face is correctly visualizing the three-dimensional structure of molecules on a two-dimensional surface, making it difficult to accurately assign R or S configurations. Employing molecular models or software that allows for the manipulation of molecular structures can be particularly helpful in mastering this skill.

Stereoisomerism - Comparing Diastereomers

The concept of stereoisomerism encompasses the various ways in which molecules with the same connectivity can differ in their spatial arrangement. Among the types of stereoisomers, diastereomers are particularly intriguing as they are not mirror images of each other. Unlike enantiomers, which are non-superimposable mirror images, diastereomers have different configurations at one or more (but not all) chiral centers.

Identifying diastereomers involves examining each stereocenter of the molecules in question. When at least one but not all of the stereocenters differ between two molecules, they are diastereomers of each other. This gives rise to different physical properties such as boiling point, melting point, and solubility, which can be crucial in the separation of these compounds in laboratory settings.

One of the exercises that students can practice is to draw out or model different stereoisomers and compare their stereocenters. By doing so, one can quickly identify pairs of molecules that are diastereomers of each other, which is an invaluable skill for chemists in the analysis and synthesis of complex organic compounds. Remember, mirror images are enantiomers, non-mirror images with different configuration at one or more stereocenters are diastereomers, and if there is no difference, they are the same molecule.