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Unit VI — Stereochemistry · Geometric & Conformational Isomerism
Rotating about single bonds gives conformers; staggered vs eclipsed, chair vs boat determine stability.
In Newman projection: staggered (60° apart) is more stable than eclipsed (0°). Barriers: staggered/eclipsed ≈ 12 kJ/mol comes from torsional strain (bonding electron pairs align).
Four conformers: anti (most stable, 0° methyls apart), gauche (60°, both + and −, ~3.8 kJ/mol higher — steric), eclipsed (120°), and fully eclipsed (0°, methyls together, ~19 kJ/mol — highest). Stability: anti > gauche > eclipsed > fully eclipsed.
Chair is strain-free (all C–C bonds staggered, angle ≈ 109.5°). Boat form is higher energy (flagpole + eclipsing). Chair has 6 axial (up/down, alternating) and 6 equatorial (around the ring) H atoms. A monosubstituted cyclohexane is more stable with the substituent equatorial (avoids 1,3-diaxial interactions).
Chair↔chair interconversion converts axial substituents to equatorial — a large group therefore lives mainly equatorial. Energy difference of t-butylcyclohexane (axial vs equatorial) ≈ 23 kJ/mol.
| Conformer | Angle | Relative energy |
|---|---|---|
| Anti | 180° | 0 |
| Gauche | 60° | 3.8 |
| Eclipsed | 120° | 16 |
| Fully eclipsed | 0° | 19 |
Config ≠ conformation. Ethane staggered > eclipsed. n-butane anti>gauche>eclipsed. Cyclohexane chair>boat (1,3-diaxial). Large substituent equatorial.