Question

Indicate the stereochemical configuration for the tetrahedral centers shown below.

Answer 1

Concepts and reason

The molecule with three different stereocentre is given and the configuration needs to be assigned. Descriptor R and S can be assigned to the centers by following the CIP rule which is based on certain criteria of giving priorities to the groups.

Fundamentals

The priority to the groups attached to the stereocentre are decided by CIP sequence rules, that is, Cahn-Ingold-Prelog rules which are as follows:

1.Firstly, locate the stereocentre in the given compound and the groups attached to it.

2.Then, compare the atomic number of the atom attached to the stereocentre. Atom having the highest atomic number assigned as the highest priority and the atom having the lowest atomic number assigned as the least priority.

3.If the first atom attached on both the sides of the stereocentre is same, then the priority is assigned to the substituent based on the next atom attached to the substituent.

4.Lowest priority group should be below the plane or away from the viewer, if is it present above the plane then change the descriptor so that the actual configuration of the molecule should not change.

For instance, if the configuration of the molecule comes out as R and lowest priority group is present above the plane then change the configuration from R to S and vice versa.

5.Then, rotation in the direction of priority order, that is, from 1 to 2 to 3 is carried out.

6.If the rotation appears as clockwise then, the stereoisomer is R. If the rotation appears as anticlockwise then, the stereoisomer is S.

Write the structure of the given molecule, assign the priority to the groups, and then write the configuration for the first site as shown below:

Hence, the configuration for the site 1 is ‘S’.

Write the structure of the given molecule, assign the priority to the groups, and then write the configuration for the second site as shown below:

Hence, the configuration for the site 1 is ‘S’.

Write the structure of the given molecule, assign the priority to the groups, and then write the configuration for the third site as shown below:

Hence, the configuration for the site 3 is ‘S’.

Ans:

The configuration at the three sites (marked as green) are as follows: