Chapter 4 : Stereochemistry B.sc 1st Year Page : 6 Organic Chemistry
Representation of 3D Molecules Absolute Configuration
The actual 3D dimensional representation of configuration of dissymmetric molecule is called Absolute configuration (3D Molecules Absolute Configuration). To specify the configuration of such dissymmetric molecule, Cahn Ingold and Prelog proposed a new system called as R-S, system or Cahn-Ingold-Prelog system or C.I.P. system. Here R stands for right Latin; rectus= right) and S stands for left (Latin; sinister = left).
In the R-S system we arrange the four different atoms or groups attached to the asymmetric carbon in decreasing order of priority(1,2,3,4,) by applying the following sequence rules.
- When the four atoms attached to the asymmetric carbon atom are all different, priority depends on atomic number. The atom who’s atomic number are higher gets the higher priority. Thus, the priority in the compound, Bromo chloro iodo methane (CH.Cl.Br.I) is :
I > Br > C > H
1 2 3 4
When the isotopes of the same element are attached to an asymmetric carbon atom „the isotope with highest mass number gets the highest priority. For example: a Deuterio ethyl bromide (CH-CH, D, Br) has the priority sequence of:
Br > CH2 > D > H
1 2 3 4
Highest priority lowest priority
- If the two atoms are identical, the atomic number of the next atoms are used for priority assignment. If it is still not possible the comparison is continued to the next atom and so on.
- Project the molecule in such a way so that the group or atom of lowest priority is at the bottom.
- If the lowest priority atom or group is not at the top or bottom the other three groups or atoms are then rotated in such a way that the lowest priority species appears at the bottom.
- Select the atoms or groups of higher priority and draw a curved arrow ( ) towards the atoms or groups of next higher priority.
- If the direction of the arrow is clockwise ( ), use the term R and if the direction of arrow is anticlockwise (n) use the term S for configuration of the dissymmetric molecule.
- The order of priority of some important groups or atoms is taken as follows:
I > Br > Cl >-SO3H > -OCOR > -OR > -OH > -NO2> -NH2 > -COOR > >CO > -CHO > -CH2OH > -CN >-CH3 > H
- A double bond is considered to be duplicated and a triple bond as triplicated. For example,
Thus ,when more than two atoms are identical the order of priority is as follow :
It is therefore to specify the R or S configuration consider the following examples :