Jawaharlal Nehru Technological University Hyderabad B. Pharmacy II Year PHARMACEUTICAL ORGANIC CHEMISTRY-III Previous Question Paper (August/September - 2022 - R17)

The R/S system (Cahn-Ingold-Prelog priority rules) is used to specify the absolute configuration of a chiral center. The rules are as follows:

1. Assign priorities to the four groups attached to the chiral center based on atomic number; the atom with the highest atomic number receives the highest priority (1), and the lowest atomic number receives the lowest priority (4).
2. If atoms are identical, compare the next atoms in the chain until a difference is found.
3. Double or triple bonds are treated as if the atoms are duplicated or triplicated.
4. Orient the molecule so that the lowest priority group (4) points away from the viewer.
5. If the sequence 1→2→3 is clockwise, the configuration is R (Rectus); if counter-clockwise, it is S (Sinister).

Optical isomerism occurs in molecules that possess a chiral center, lack a plane of symmetry, and exist as non-superimposable mirror images called enantiomers. These isomers rotate the plane of plane-polarized light in opposite directions (dextrorotatory or levorotatory) but possess identical physical and chemical properties in an achiral environment.

Meso compounds are molecules that contain multiple chiral centers but are achiral overall due to an internal plane of symmetry. Because of this symmetry, the optical rotation caused by one half of the molecule is cancelled by the other half, rendering the entire molecule optically inactive. A classic example is meso-tartaric acid, which has two chiral centers but is superimposable on its mirror image.

Chiral molecules are those that lack any internal elements of symmetry (plane of symmetry, center of inversion, or improper axis of rotation) and are non-superimposable on their mirror images. Chirality is a property of the molecule as a whole. For example, lactic acid (2-hydroxypropanoic acid) is a chiral molecule because the central carbon is bonded to four distinct groups (H, OH, CH3, COOH), creating a stereocenter that allows for two distinct enantiomeric forms.