R S configuration Organic Chemistry | optical isomerism trick | IIT JEE & NEET | Vineet Khatri Sir

Summary

In this video, the presenter introduces the concept of R and S configurations in organic chemistry, focusing on the assignment of configurations at chiral centers. The tutorial explains how to determine R and S configurations using the CIP (Cahn-Ingold-Prelog) rules, emphasizing the importance of atomic number and the positioning of groups around the chiral center. Several examples, including Fisher projections and different molecular arrangements, are discussed to help students understand how to navigate the process. The presenter also emphasizes the role of practicing these concepts to master complex questions and improve understanding in organic chemistry.

Takeaways

• 😀 The R and S configuration in organic chemistry refers to the 3D arrangement of groups around a chiral center.
• 😀 The priority of groups attached to a chiral center is determined using the Cahn-Ingold-Prelog priority rules, where the atomic number plays a key role.
• 😀 R configuration is clockwise, while S configuration is counterclockwise when the lowest priority group is placed behind the plane of the molecule.
• 😀 The correct configuration is determined by observing the direction of the priority groups and applying the rules for the chiral center.
• 😀 If the lowest priority group (LP) is positioned in the plane or above it, the configuration may need to be reversed to give the correct R or S assignment.
• 😀 The script emphasizes that the lowest priority group should always be in the plane or below it for proper assignment of the R or S configuration.
• 😀 If the lowest priority group is on the horizontal plane, the configuration needs to be reversed after observing the sequence of groups.
• 😀 The process involves assigning priorities to groups, drawing lines to connect the groups, and analyzing the direction (clockwise or counterclockwise) of the sequence.
• 😀 The script also demonstrates how the configuration changes when there are multiple chiral centers in a molecule, making use of Fischer projections for clarity.
• 😀 Practice is emphasized, as understanding the principles of R and S configuration requires solving multiple examples to reinforce the concepts.

Q & A

What does the R and S configuration in organic chemistry refer to?

The R and S configuration refers to the 3D spatial arrangement of atoms around a chiral center in a molecule. The R configuration is assigned when the arrangement is clockwise, and the S configuration is assigned when the arrangement is counterclockwise.

What is a chiral center?

A chiral center is a carbon atom bonded to four different atoms or groups, resulting in non-superimposable mirror images, known as enantiomers.

How do you determine the priority of groups attached to a chiral center?

The priority is determined by the atomic number of the atoms directly bonded to the chiral center. The atom with the higher atomic number gets the higher priority. If two atoms are the same, you compare the atomic numbers of the next set of atoms bonded to them.

What does the 'R' configuration signify in terms of atom arrangement?

The 'R' configuration signifies a clockwise arrangement of the atoms or groups around the chiral center when observed from a specific direction, following the priority rules.

What does the 'S' configuration signify?

The 'S' configuration signifies a counterclockwise arrangement of the atoms or groups around the chiral center when observed from a specific direction, following the priority rules.

What is the significance of the lowest priority group in determining R/S configurations?

The lowest priority group must be placed in a position that is behind the plane of the chiral center (in the 'wedge' or 'dash' position). If it is not, the R/S configuration must be reversed to obtain the correct result.

How does the presence of a wedge or dash affect the determination of R/S configuration?

The wedge indicates a group coming out of the plane (toward the observer), and the dash indicates a group going into the plane (away from the observer). For correct R/S determination, the lowest priority group should be in the dash position. If it is not, the configuration result needs to be reversed.

What happens if the lowest priority group is not in the correct position (i.e., not in the dash position)?

If the lowest priority group is not in the correct position (behind the plane), the configuration needs to be reversed. For example, if the configuration seems to be R but the lowest priority group is not in the dash position, the answer would be S.

What is the 'Cahn-Ingold-Prelog' system used for?

The Cahn-Ingold-Prelog system is used to assign priorities to the groups attached to a chiral center based on atomic numbers and the structure of atoms or groups. This system helps in determining the correct R/S configuration.

How are configurations determined in molecules with multiple chiral centers?

In molecules with multiple chiral centers, each chiral center's configuration (R or S) is determined individually by applying the priority rules to the groups attached to it. The configurations of all centers must be determined to understand the overall stereochemistry of the molecule.