Stereoisomers
High-Yield Summary
- Stereoisomers share atomic connectivity but differ in spatial arrangement: conformational isomers (interconvert by single-bond rotation) vs. configurational isomers (require breaking bonds).
- Newman projections view a molecule down a C-C bond (front carbon = dot, back = circle); dihedral angle and torsional strain describe substituent interactions across that bond.
- Butane conformations, most to least stable: anti (180°) < gauche (60°) < eclipsed (120°) < totally eclipsed (0°) [energy increasing in that order].
- Cyclohexane's chair conformation minimizes angle strain, torsional strain, and nonbonded strain — most stable ring conformation; substituents prefer equatorial over axial to avoid 1,3-diaxial interactions; ring flip converts every axial ↔ equatorial.
- A chiral center = carbon bonded to 4 different groups. x chiral centers → up to 2ˣ stereoisomers (meso compounds are the key exception). CIP rules assign R/S; enantiomers/diastereomers/meso compounds classify stereoisomer pairs.
Must Know
- Anti is lowest energy (no steric/torsional strain); totally eclipsed is highest energy (max steric + torsional strain).
- Bulky substituents (e.g. tert-butyl) strongly prefer equatorial in the chair conformation to avoid 1,3-diaxial interactions with axial hydrogens.
- 1 chiral center → enantiomers possible. 2+ chiral centers → enantiomers and diastereomers both possible.
- Enantiomers: identical properties in achiral environments, differ only in plane-polarized light rotation and reactivity with other chiral molecules.
- Diastereomers have genuinely different physical/chemical properties even in achiral environments — separable by ordinary methods (unlike enantiomers).
- Meso compound = even number of chiral centers + internal plane of symmetry + opposite (R/S) configurations at every center → overall achiral despite chiral centers.
Assigning R/S with the CIP Rules
- 1Identify the four groups directly attached to the chiral center.
- 2Assign priority by atomic number of the directly bonded atom — highest = Priority 1, lowest (usually H) = Priority 4. Break ties by moving outward bond by bond; double bonds count as a phantom-duplicate atom.
- 3Orient the molecule so Priority 4 points directly away from you (on a dash).
- 4Trace Priority 1 → 2 → 3: clockwise = R, counterclockwise = S.
- 5Correction if Priority 4 isn't already on a dash: on a dash → no adjustment. On a wedge/in-plane → flip the traced answer (R↔S). Opposite the dash (on a solid wedge) → no adjustment.
Classifying a Stereoisomer Pair (2 Chiral Centers)
| R/S Comparison at C2 and C3 | Relationship |
|---|---|
| Opposite at both centers | Enantiomers (mirror images) |
| Same at one, different at the other | Diastereomers |
| Same at both | Identical molecules |
| Opposite R/S at both + plane of symmetry | Meso compound (achiral) |
Key Terms
- Newman projection
- View down a C-C bond axis; front carbon = dot (substituents radiate from it), back carbon = circle (substituents radiate from its edge).
- Dihedral angle (θ) / Torsional strain
- Angle between a front-carbon and back-carbon substituent in a Newman projection; torsional strain is repulsion between overlapping adjacent bond electron clouds, raising energy.
- Ring strain (angle / torsional / nonbonded)
- Angle strain: bond angles off ideal. Torsional strain: eclipsed/gauche interactions within the ring. Nonbonded (steric) strain: nonadjacent groups crowding, especially bulky substituents.
- Axial vs. equatorial
- In the chair conformation, axial positions point straight up/down (alternating per carbon); equatorial positions point outward along the ring plane, always opposite direction to that carbon's axial position.
- Chiral center
- A carbon bonded to four different groups — the essential first thing to identify in any stereoisomer analysis.
- Specific rotation
- [α] = α_obs / (c × l) — α_obs = observed rotation (degrees), c = concentration (g/mL), l = path length (dm); quantifies how much an enantiomer rotates plane-polarized light.
Common MCAT Trap
- Don't assume x chiral centers always gives exactly 2ˣ distinct stereoisomers — meso compounds reduce that count by making two 'different' R/S combinations the same achiral molecule.
- Enantiomers look chemically identical in ordinary (achiral) lab conditions — they only reveal themselves via plane-polarized light or reaction with another chiral species. Don't expect a boiling-point or solubility difference.
- A molecule can have chiral centers and still be achiral overall (meso) — always check for an internal plane of symmetry before concluding a molecule is chiral just because it has stereocenters.
Quick Recall
Rank butane's four conformations from most to least stable.
Why do bulky substituents prefer the equatorial position on cyclohexane?
In the CIP method, what do you do if Priority 4 is on a wedge instead of a dash?
What three features together identify a meso compound?
How do enantiomers and diastereomers differ in physical properties in an achiral environment?