JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II PHARMACEUTICAL ORGANIC CHEMISTRY-III Previous Question Paper (August- 2021 - R17)

A racemic modification, also known as a racemic mixture or racemate, is an equimolar mixture of two enantiomers of a chiral compound. Because the mixture contains equal amounts of the dextrorotatory (+) and levorotatory (-) forms, the optical rotations of the two enantiomers cancel each other out, resulting in an optically inactive substance with a net optical rotation of zero.

Racemic modifications are designated by the prefix (±) or (dl). While they share the same physical properties as the individual enantiomers (such as boiling point, melting point, and solubility), they may differ in their crystalline structure, density, and solubility compared to the pure enantiomers. The process of forming a racemic mixture from a pure enantiomer is known as racemization.

Resolution is the process of separating a racemic mixture into its individual pure enantiomers. Since enantiomers have identical physical properties in an achiral environment, standard techniques like distillation or crystallization are ineffective. Common methods include:

1. Mechanical Separation: Applicable only when the enantiomers crystallize as separate, visually distinguishable crystals (conglomerates), as seen in Pasteur's classic separation of sodium ammonium tartrate.
2. Chemical Resolution (Diastereomeric Salt Formation): The most widely used method. The racemic mixture is reacted with a pure chiral resolving agent (e.g., an optically active acid or base) to form a pair of diastereomeric salts. Unlike enantiomers, diastereomers have different physical properties (solubility) and can be separated by fractional crystallization. Once separated, the individual enantiomers are recovered by treating the salts with an acid or base.
3. Biochemical Resolution: Utilizes microorganisms or enzymes that selectively metabolize or transform one enantiomer while leaving the other unchanged. This method is highly specific but often results in the loss of one enantiomer.
4. Chromatographic Resolution: Employs chiral stationary phases (CSPs) in column chromatography. The enantiomers interact differently with the chiral environment of the stationary phase, leading to different elution times and enabling effective separation.