JNTU B.Pharmacy II Year PHARMACEUTICAL ORGANIC CHEMISTRY-III Previous Question Paper (March/April-2021 - R17)

Optical isomerism is a form of stereoisomerism where molecules have the same molecular formula and connectivity but differ in their spatial arrangement, resulting in the ability to rotate the plane of plane-polarized light. This property arises in molecules containing chiral centers.

Enantiomers are non-superimposable mirror images of each other. For example, Lactic acid exists as two enantiomers, (+)-lactic acid and (-)-lactic acid, which rotate plane-polarized light in opposite directions.

Diastereoisomers are stereoisomers that are not mirror images of each other. This typically occurs in molecules with more than one chiral center. For example, Tartaric acid has three stereoisomers: (R,R)-tartaric acid, (S,S)-tartaric acid, and the meso form. The (R,R) and (R,S) forms are diastereomers.

A Meso compound is an optically inactive member of a set of stereoisomers that contains chiral centers but possesses an internal plane of symmetry, causing the rotation of one half of the molecule to be cancelled by the other. Meso-tartaric acid is the classic example, where the internal symmetry renders the molecule achiral despite having two chiral carbon atoms.

The DL system (or Fischer-Rosanoff convention) is used to specify the configuration of chiral molecules by comparing them to the reference standard, glyceraldehyde.

In this system, the molecule is drawn in a Fischer projection with the most oxidized carbon (e.g., aldehyde or carboxyl group) at the top. The configuration is determined by the position of the hydroxyl (-OH) or amino (-NH2) group attached to the chiral carbon furthest from the top:

• D-configuration: The functional group (-OH or -NH2) is on the right side of the carbon chain.
• L-configuration: The functional group (-OH or -NH2) is on the left side of the carbon chain.

For example, in D-glyceraldehyde, the -OH group is on the right. Similarly, in L-alanine, the amino group (-NH2) is positioned on the left side of the chiral carbon. This system is widely used in biochemistry for naming carbohydrates and amino acids, though it does not directly correlate to the direction of optical rotation (+ or -).