Stereochemistry

The core idea

Stereoisomers have the same connectivity (same bonds, same molecular formula) but differ in the arrangement of atoms in space. They split into two families: enantiomers (non-superimposable mirror images) and diastereomers (stereoisomers that are not mirror images).

Chirality & stereocenters

• A molecule is chiral if it is non-superimposable on its mirror image. The reliable test: no improper axis of symmetry (no plane σ, no centre i, no S_n).
• A stereocenter (chiral centre) is typically an sp³ carbon bearing four different groups.
• A molecule can have stereocenters yet be achiral (a meso compound) because of internal symmetry.
• Chirality can also arise without a stereocenter — e.g. allenes, biphenyls (atropisomerism), helicenes.

R/S — the CIP rules

• Rank the four groups by CIP priority: higher atomic number = higher priority (at the first point of difference).
• Ties are broken by moving outward atom-by-atom to the first point of difference.
• Double bonds are treated as duplicate (phantom) atoms: C=O counts as C bonded to (O, O).
• Point the lowest priority (4) away from you; 1→2→3 clockwise = R, counter-clockwise = S.
• If lowest priority points toward you, determine the rotation and then invert the answer.

E/Z — double-bond configuration

• Assign CIP priority to the two groups on each alkene carbon.
• Higher-priority groups on the same side → Z (zusammen, together). Opposite sides → E (entgegen).
• E/Z is the rigorous replacement for cis/trans (cis/trans is ambiguous when the carbons carry unlike groups).

Enantiomers vs diastereomers

• Enantiomers: mirror images; identical physical properties (mp, bp, solubility) except they rotate plane-polarized light in opposite directions and react differently with other chiral things.
• Diastereomers: not mirror images; different physical properties → separable by ordinary methods (distillation, crystallization, chromatography).
• For n stereocenters, maximum stereoisomers = 2ⁿ (fewer if meso forms exist).

Meso compounds

• A meso compound contains stereocenters but is achiral — it has an internal mirror plane (or centre of symmetry) so it is superimposable on its mirror image.
• Classic example: meso-tartaric acid — (R,S) with an internal σ plane; it is optically inactive (internal compensation).

Optical activity & specific rotation

• Dextrorotatory (+, d) rotates light clockwise; levorotatory (−, l) counter-clockwise.
• Sign of rotation (+/−) is experimental and has no fixed relationship to the R/S label.
• Enantiomeric excess: % ee = (observed [α] / [α] of pure enantiomer) × 100.

Fischer / Newman / wedge — reading structures

• Wedge–dash: bold wedge = toward viewer, dashed = behind.
• Fischer: vertical bonds go back, horizontal bonds come forward; carbon chain vertical, most-oxidized carbon on top.
• Newman: view down a C–C bond; front atom = dot, back atom = circle. Used for conformations (staggered / eclipsed, gauche / anti).

Racemic mixtures

• A racemate (racemic mixture, ±) is a 1:1 mix of two enantiomers → optically inactive (external compensation).
• Separating a racemate into single enantiomers = resolution (e.g. via diastereomeric salts).

30-second recall table