Question

What is the R,S configuration for the following structure of isocitric acid?

1. 2-R, 3-S

2. 2-S, 3-R

3. 2-R, 3-R

4. 2-S, 3-S

Answer 1

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

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

2. Then, compare the atomic number of the atom attached to the stereo centre. 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 stereo centre 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.

5. In fisher projection, the lowest priority group should be present on the vertical line. If lowest priority group is present on the horizontal line then change the stereochemistry of the chiral centre.

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

7. If the rotation appears as clockwise then, the stereoisomer is \(\mathrm{R}\). If the rotation appears as anticlockwise then, the stereoisomer is S.

The given molecule is as follows: Но ОН 4 ОН isocitric acid The groups attached to the chiral centre 2 present in the reactant given are as follows: ОН, Н, —СООН, and C —CООН Arrange them in decreasing order of their atomic number according to the first atom of the substituent as shown below: ОН > -СООН >С-СООН > Н As the order of priority is directly related to the atomic number therefore, order of priority will be as follows: ОН >-СООН >С-СООН >Н

Highest priority group is marked as the lowest number, that is, 1 as shown in the structure and then rotation is carried out in the direction of priority, that is, from 1 to 2 to 3 as shown below: СООН НО1 The rotation appears anticlockwise; therefore, the configuration should be S but H, that is, the lowest priority group is present below the plane, thus, the configuration is reversed. Hence, the configuration is R for the stereo centre present in the given reactant.

The given molecule is as follows: HO 3 НО "ОН 4 ОН isocitric acid The groups attached to the chiral centre 3 present in the reactant given are as follows: Н, -СООН, С -ОН and-CН, Arrange them in decreasing order of their atomic number according to the first atom of the substituent as shown below: -COOH > C- OH >-CH, > H As the order of priority is directly related to the atomic number therefore, order of priority will be as follows: —СООН > С-ОН >-СН,> H

Highest priority group is marked as the lowest number, that is, 1 as shown in the structure and then rotation is carried out in the direction of priority, that is, from 1 to 2 to 3 as shown below: НООС, 3 2 CH2 ÓH The configuration is S for the stereo centre present in the given reactant. Therefore, the R, S configuration is 2-R, 3-S. The correct option is 1.