Nomenclature of optical isomers: D&L system and R&S system, with best notes(1)

Nomenclature of optical isomers

1. D&L system of nomenclature of optical isomers

The definition of R&S configuration is more precise & quiet different D&L configuration. The D&L nomenclature was done in old system. The original system of nomenclature was started with the enantiomers of a standard compound glyceraldehyde one enantiomers rotate the plane of polarized light to the right or clockwise & it was referred to as (+) glyceraldehyde while the other enantiomers rotated the plane polarized light to the left or counter clockwise & was referred to as (-) glyceraldehyde can be represented with the fisher projective. Nomenclature of optical isomers

Example:- configuration of (-) lactic acid is related to D (+) glyceraldehyde thus (-) lactic acid belongs to D series has the following configuration

All other compounds containing asymmetric carbon could be related one or other form of the glyceraldehyde & thus there configuration are determined. Compound where configuration are related to D (+) glyceraldehyde are taken belongs to D-series and those related to (-) glyceraldehyde belongs to L-series. Nomenclature of optical isomers

Sequence rules for DL system:

1. Draw proper fischer projection of molecules.

2. Highest oxidised group must be on top. (If not then rearrange this).

3. Observe bottom most chiral (assymetric) carbon, if it contain -OH/-NH2 at right “D”, if at left than “L”.

4. Longest chain is always on vertical line.

5. If -OH and -NH2 both are present than D/L are decided by -NH2. group.

2. R&S system of nomenclature of optical isomers

This configuration is given by three scientists (European) R.S cahn, C.K. ingold and V. Prelog. It is also known as “CIP system” (Cahn Ingold prelog). Two steps are involved for nomenclature of optical isomers.

Step-1:- we assigna sequence of priority to the 4 atoms or group attached to the chiral centre. Ex- Bromochloroiodomethane.

Step-2:- we visualize the molecule oriented so that the ligand of lowest priority is directed away from us & above the arrangement of remaining object in proceeding from the ligand of highest priority to the ligand of second priority & than 2 the 3, our eye travel in clockwise direction, the configuration is specialized ‘R’ [latin term ‘Ractus’ means right’], if anticlockwise the configuration is specified ‘S’ [latin term ‘sinister means left’].

Sequence rule of R S system

Rule-1:-

If 4 atoms attached to the chiral centers are all different priority will depend in the atomic number with the atom of higher atomic number getting higher priority. If the two atoms are isotopes of the same element, the atom of higher mass number has the priority. Example– chloroidomethane sulphuric acid, α-Deuterioethyl bromide.

Rule-2:-

If the relative priority of the 2 groups can’t be decided by the rule 1, then it is determined by similar comparison of next atoms in the groups. Here, CN₃ & C₂H₅ atoms contains carbon atom directly attached to the chiral center thus in CH₃ the second atoms will be H>H>H while in C₂H₅, they are H>H>C. Since carbon has a higher atomic number over hydrogen so the sequence of the structure will be Cl>C₂H₅>CH₃>H.

Rule-3:-

When their is a double bond or triple bond, both atoms are considered to duplicated or triplicated.