Difference Between R and S Configuration

The key difference between R and S configuration is that the R configuration is the spatial arrangement of R isomer, which has its relative direction of priority order in a clockwise direction whereas S configuration is the spatial arrangement of S isomer that has its relative direction of priority order in an anticlockwise direction. Here, the relative direction of the priority order is the descending order of priorities of substituents.

The R and S isomers are organic molecules having a chiral center, which is a carbon atom that has four different substituents attached to it. These substituents get listed according to their priority (the priority is determined using CIP rules as described below).

1. Overview and Key Difference
2. What is R Configuration
3. What is S Configuration
4. Similarities Between R and S Configuration
5. Side by Side Comparison – R vs S Configuration in Tabular Form
6. Summary

What is R Configuration?

An isomer is each of two or more compounds with the same formula but a different arrangement of atoms in the molecule. R configuration is the spatial arrangement of R isomer. Thus, R isomer has its relative direction of the priority order in a clockwise direction. The basis for determining the priority of substituents attached to the chiral center is the CIP rules (Cahn-Ingold-Prelog rules). The CIP rules are as follows:

First, consider the atoms that directly bond to the chiral center. When the atomic number is higher, the priority of it also gets higher. Therefore, if a substituent contains an atom with a high atomic number bonded directly to the chiral center, that substituent gets the higher priority than the others.
If two substituents have directly bonded atoms with equal atomic numbers, then consider the atomic number of the next atom in those substituents. We have to check the atoms of the substituents one by one until a point of difference comes.

Difference Between R and S Configuration

Figure 01: R and S Configurations

After determining the priorities of each substituent, we should observe the direction of the order of the priorities around the chiral center; that is, from the highest priority to the lowest priority substituent. If the direction is clockwise, then the configuration of the isomer is named as R configuration. The letter “R” comes from the Latin word “Rectus”. It means, “Right-handed”.

What is S Configuration?

S configuration is the spatial arrangement of S isomer. S isomer has a different arrangement unlike that of the R isomer of the same molecule. The letter “S” comes from the Latin word “Sinister”, and it means, “Left-handed”. Unlike R configuration, the S configuration has the anticlockwise direction of the substituents; that is, from the highest priority to the lowest priority.

What are the Similarities Between R and S Configuration?

Both R and S Configuration have the same chemical formula and atomic arrangement
Both have the similar molar masses.

What is the Difference Between R and S Configuration?

R vs S Configuration
R configuration is the spatial arrangement of R isomer.	S configuration is the spatial arrangement of S isomer.
Priority of Substituents
R isomer has its relative direction of the priority order in the clockwise direction.	S isomer has its relative direction of the priority order in an anticlockwise direction.
Name
The letter “R” comes from the Latin word “Rectus” meaning, “Right-handed”.	The letter “S” comes from the Latin word “Sinister”. It means, “Left-handed”.
Spatial Arrangement
The spatial arrangement of R configuration is different from that of the S configuration of the same molecule.

Summary – R vs S Configuration

The organic compounds having chiral centers have R and S configurations. The R and S isomers are the related molecules of these configurations, respectively. The basis of R and S configuration is the priority of the substituents attached to the chiral center. To sum up the comparison; the difference between R and S configuration is that the R isomer has its relative direction of the priority order in a clockwise direction. And, in contrast, the S isomer has its relative direction of the priority order in an anticlockwise direction.

Reference:

1. Libretexts. “Absolute Configuration: R-S Sequence Rules.” Chemistry LibreTexts, Libretexts, 6 Nov. 2017. Available here
2. Hunt, Ian R. R/S Nomenclature. Available here
3. “Absolute Configuration.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018. Available here