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Unit VI — Stereochemistry · Optical Isomerism
Molecules with two stereogenic centres show enantiomers, diastereomers and the special meso form.
2,3-dihydroxybutanedioic acid has 3 stereoisomers: (R,R), (S,S) — enantiomers forming a pair; and meso-tartaric acid (R,S = S,R, internally compensated, optically inactive). The meso form has an internal plane of symmetry.
For two centres, when the two highest priority groups are on the same side in a Fischer projection the pair is called 'erythro'; on opposite sides, 'threo'. (Each of the enantiomer pairs is either threo or erythro.)
Separating the two enantiomers of a racemic mixture. Methods: (a) mechanical separation of crystals, (b) biochemical (microorganism consumes one enantiomer), (c) formation of diastereomeric salts with an optically active resolving agent followed by fractional crystallization (the most common chemical method).
SN2 inverts configuration (Walden inversion); SN1 tends to racemize (attack from both faces); retention preserves configuration (e.g., substitution without breaking the stereogenic bond, or with anchimeric assistance).
| Case | Relationship |
|---|---|
| (R,R) vs (S,S) | Enantiomers |
| (R,R) vs (R,S) | Diastereomers |
| (R,S) alone when symmetric | Meso — optically inactive |
2ⁿ rule; diastereomers = non-mirror stereoisomers; meso = inactive by internal compensation; resolution via diastereomeric salts.