Chemical Science Short Notes: R/S Nomenclature (Download PDF)

By Renuka Miglani|Updated : March 15th, 2023

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Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on R/S Nomenclature, which comes under the Organic Chemistry section of the Chemical Science syllabus.

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Study Notes on R/S Nomenclature (Download PDF)

This is an absolute configuration of the chiral center proposed by R.S. Chan, Sir Christopher Ingold, and V. Prelog. It was derived to provide exact knowledge and differentiation among the enantiomers.

To start with the nomenclature, we first need to find a carbon atom attached to four different groups, known as Chiral Centre and then we follow the rules.

Sequence rule: Priority to the four atoms or groups of atoms attached to the chiral center can be determined by following certain sequence rules:

Rule 1: If the chiral center is attached to four completely different groups, then priority depends on atomic number. The atom of a high atomic number gets the higher priority.

Rule 2: In the case of isotopes, the atom with a higher atomic mass number gets the higher priority.

For example: among Br, C, D, and H priority order is

Br > C > D > H.

Rule 3: If the relative priority of two groups cannot be decided by the rule mentioned above, then we move towards the next atoms in sequence.

Rule 4: Where there is a double bond or triple bond, both atoms are considered to be duplicated or triplicated; as shown below -

(1) Using the above-given method, assign the priority.

(2) Find the position of the 4th group.

(3) Connect 1 → 2 → 3 making a circle. And see how it goes- clockwise or anticlockwise.

Case 1: If the fourth group is below the plane.

Note: In this case 4th group 'H' is below the plane and rotation is clockwise. So, it is 'R'.

Case 2: If the fourth group is above the plane.

Then,

Note: In this case, the 4th group is above the plane. So, on moving clockwise we obtain 'S'.

Case 3: If the fourth group is on the plane then make a double interchange in such a way that the 4th group goes below the plane.

So, in compounds ‘A’ and ‘B’, the configuration at the chiral center are

Diastereomers:

If structures have more than one stereogenic centre, it gives rise to Diastereomers.

Example:

Meso Compounds:

Compounds that contain stereogenic centers but are achiral are known as meso compounds. There is a plane of symmetry with ‘R’ stereochemistry on one side and ‘S’ stereochemistry on another.