RS system of nomenclature of optical isomers)

RS system of nomenclature of optical isomers

Robert Sidney Cahn, Christopher Ingold, and Vladimir Prelog, developing a nomenclature system in 1956, led to the assignment of chirality centers in molecules to their absolute configurations. It provides accurate and unambiguous names for chiral molecules regardless of the presence of multiple asymmetric centers when the RS nomenclature system is added to the IUPAC nomenclature system.


As light passes through a solution containing chiral. molecules, they usually rotate plane-polarized light. Rotation of plane-polarized light does not reveal the RS configuration of chiral compounds; thus, it should be emphasized that this is not true. It is also possible to describe the structure of chiral molecules using the Fischer-Rosanoff convention. This system labels the entire molecule rather than each chirality center, and often returns ambiguous results when describing molecules with more than one chirality center.

Priority Rules of RS System

There is a curved arrow to indicate whether the chirality center is R or S, depending on the priority sequence assigned to the group attached to it.

First Rule

On the basis of the atomic number of the atom connected directly to the chiral center, priority sequence is assigned to each group.


• Atoms with the highest atomic number are given priority.


• Atoms with the lowest atomic number are given the lowest priority.

Accordingly, an oxygen atom, O, with an atomic number of 8, a carbon atom, C, with an atomic number of 6, a chlorine atom, Cl, with an atomic number of 17, and a bromine atom, Br, with an atomic number of 35 are attached to the chiral center in this manner: Br> CI > O > C. Isotopes are ranked according to atoms with the highest atomic mass.

Second Rule

In order to determine the priority sequence, the atomic number of the next atom bound to the chirality center is used, and this number is increased from the center until the first point of difference is reached. In other words, different groups attached to the chirality center by the same atoms are assigned different priorities based on the atomic number. A chiral center consists of three groups attached to the same atom: -CH3, -CH2CH3, and -CH2OH. When we examine the next atom bound, we



As oxygen has a higher atomic number than carbon, and carbon is higher than hydrogen, oxygen comes first in the priority order


CH2OH CH2CH3-CH3

Some groups are listed in priority order as follows:


-I > -Br>-CI > -SH-OR-OH-NHR > -NH2>


- COOR>-C00H-CHO > -CH2OH>-C6H5 > -CH3 > - 2H >-1H

A chirality center cannot be chiral if the groups attached to it have similar priority rankings. In order to align molecules in space properly, the lowest priority. group must face away from the viewer, then be situated behind the chiral center. Make a circle from the highest to the lowest priority group by tracing a curved arrow.

• A clockwise rotation yields R as the configuration of the chiral center, from the Latin rectus, which means right.

The chiral center of a circle is shaped like an S, based on the Latin sinister, which means "left



As indicated in rule three of the RS system, when chirality centers have double or triple bonds in their groups, we can determine the configuration of the chirality center. Assigning priorities to atoms with double and triple bonds requires the consideration of duplicates and triples.





Double bonds are formed when one Y atom is attached to one carbon atom, and the other way around. A carbon atom is connected to three Y atoms at the same time a Y atom is connected to a carbon atom. This is known as a C = Y triple bond.