Stereochemistry
Stereochemistry is about 3D information: it controls biological activity and many mechanism outcomes. The core skills: R/S, E/Z, conformations, and stereospecific vs stereoselective reactions.
Assigning R/S (CIP rules)
- Rank substituents by atomic number at the first point of difference.
- If tied, look outward until a difference appears (treat multiple bonds as duplicated atoms).
- Orient the lowest priority group (#4) away from you.
- Trace 1→2→3: clockwise = R, counterclockwise = S.
Mechanism link: SN2 gives inversion at the reacting stereocenter.
E/Z for alkenes
Use CIP priorities on each alkene carbon: if high-priority groups are on the same side → Z, opposite → E.
| Topic | Key note |
|---|---|
| cis/trans vs E/Z | cis/trans works only for simple cases; E/Z is general |
| Stability | often E (trans-like) is more stable due to less steric strain |
| Additions | anti vs syn addition affects relative stereochemistry |
Conformations (Newman & chair)
Conformations interconvert without breaking bonds. They control rates and selectivity (e.g., E2 requires anti-periplanar geometry).
Butane Newman (energy vs dihedral)anti lowest, eclipsed highest
Cyclohexane chair ideaaxial vs equatorial
Stereospecific vs stereoselective
| Term | Meaning | Example |
|---|---|---|
| Stereospecific | different stereoisomer reactants give different stereoisomer products | SN2 inversion; anti bromination of alkenes |
| Stereoselective | one stereoisomer product favored from the same reactant | hydrogenation often gives syn addition; E2 favors anti-periplanar elimination |