R/S Configuration
CIP-based descriptors assigning absolute configuration to stereocenters.
Context: Universal language for stereochemical specification in pharma and patents.
Example: S-ibuprofen is the active eutomer.
Related Terms: CIP Rules, Absolute Configuration.
Reference: Cahn–Ingold–Prelog rules (1956/1966).
Racemate (rac)
A 1:1 mixture of enantiomers.
Context: Common synthetic outcome; may require resolution or chiral switching.
Example: Racemic thalidomide.
Related Terms: Conglomerate, Racemic Compound.
Reference: FDA Policy on Stereoisomeric Drugs (1992).
Racemic Compound
Racemate that crystallizes as a single racemic crystal containing both enantiomers.
Context: Limits simple preferential crystallization; requires alternative resolution routes.
Example: Racemic mandelic acid (context dependent).
Related Terms: Racemate, Conglomerate.
Reference: Eliel & Wilen (1994).
Racemization
Conversion of an enantiopure substance to a racemate.
Context: Critical risk for chiral APIs and must be controlled in process/storage.
Example: Base-catalyzed racemization of α-chiral carbonyls.
Related Terms: Enantiomerization, Stereomutation.
Reference: IUPAC Gold Book.
Raman Optical Activity (ROA)
Difference in Raman scattering for circularly polarized light.
Context: Complementary to VCD; sensitive to conformation of chiral molecules and biomacromolecules.
Example: ROA of proteins to assess secondary structure.
Related Terms: VCD, CD.
Reference: Barron, Molecular Light Scattering (2004).
Re/Si Face
Descriptors for the two faces of a trigonal (sp²) center.
Context: Used to define enantioface selectivity in additions and reductions.
Example: Hydride attack on the Re face of a ketone.
Related Terms: Prochirality, Pro-R/Pro-S.
Reference: IUPAC Gold Book.
Relative Configuration
The spatial relationship between stereocenters within a molecule without reference to absolute R/S.
Context: Useful for assigning diastereomer series in synthesis and NMR analysis.
Example: Erythro vs threo in sugars.
Related Terms: Diastereomer, Absolute Configuration.
Reference: Eliel & Wilen (1994).
Relative Stereochemistry
Definition: The spatial relationship between stereocenters within a molecule without reference to absolute configuration.
Context: Important for describing diastereomers in synthesis and structure elucidation.
Example: Erythro and threo configurations of 2,3-butanediols.
Related Terms: Absolute Configuration; Diastereomer; Enantiomer.
Reference: Eliel & Wilen (1994).
Resolution
Definition: The process of separating a racemic mixture into its individual enantiomers.
Context: A key step in the preparation of enantiopure compounds for pharmaceutical use. Can be achieved through crystallization, chemical or enzymatic methods, or chiral chromatography.
Example: Resolution of racemic mandelic acid using (+)-?-phenylethylamine.
Related Terms: Racemate; Enantiomeric Excess; Chiral HPLC; Kinetic Resolution.
Reference: Jacques, J., Collet, A. & Wilen, S. H. Enantiomers, Racemates, and Resolutions. Wiley, 1981.