Overview
This lecture covers the stability and conformations of cyclic alkanes, focusing on the structural features and relative stabilities of cyclopropane, cyclobutane, cyclopentane, and especially cyclohexane.
Alkane and Cyclic Alkane Stability
- Straight-chain alkanes are most stable in a zigzag, staggered conformation due to minimized steric strain and maximized electron delocalization.
- In sp³-hybridized carbons, the ideal bond angle is 109.5°.
- Early chemists (e.g., Adolf von Baeyer) thought all cyclic compounds were planar, predicting their stability by comparing bond angles to the ideal.
Angle Strain and Planarity in Small Rings
- Angle strain results from bond angles deviating from 109.5°, causing poor orbital overlap and weaker, less stable bonds.
- Cyclopropane has 60° bond angles, causing high angle strain and all hydrogens are eclipsed, further destabilizing the molecule.
- Cyclobutane has 90° bond angles (less angle strain than cyclopropane) but still has many pairs of eclipsed hydrogens.
- Both rings are not planar in reality; they twist to reduce eclipsed hydrogens, accepting some angle strain to increase stability.
Cyclopentane and Cyclohexane: Stability and Conformations
- Planar cyclopentane would have nearly ideal bond angles but too many eclipsed hydrogens; it adopts a non-planar shape to minimize eclipsing and angle strain.
- Cyclohexane is the most stable cyclic alkane, often existing in a non-planar chair conformation.
- The chair conformation of cyclohexane eliminates angle strain (bond angles ≈111°) and places all hydrogens in staggered positions.
Cyclohexane Conformations: Chair, Boat, and Ring Flip
- Chair conformation is the most stable due to no eclipsed hydrogens and minimal angle strain.
- Each carbon in the chair has one axial (up/down) and one equatorial (outward-slanting) bond.
- Cyclohexane can undergo a “ring flip” interconversion, swapping axial and equatorial positions.
- Substituents prefer equatorial positions to minimize steric strain when present.
- Other conformations: the boat has some eclipsed hydrogens and flagpole interactions (less stable), twist-boat is more stable than boat, and half-chair is least stable.
Key Terms & Definitions
- Steric strain — Repulsion between atoms/groups due to close proximity.
- Angle strain — Weakening of bonds from bond angles deviating from their ideal values.
- Eclipsed hydrogens — Hydrogens lined up with each other, increasing torsional strain and instability.
- Chair conformation — The most stable 3D shape for cyclohexane, free of angle and torsional strain.
- Boat conformation — A less stable cyclohexane form with some eclipsed hydrogens and flagpole strain.
- Axial bond — Bond perpendicular to the average plane of the ring (up or down).
- Equatorial bond — Bond extending outward, around the edge of the ring.
- Ring flip — Process where chair conformers of cyclohexane interconvert, swapping axial and equatorial positions.
Action Items / Next Steps
- Build physical models of cyclohexane using a model kit to practice making chair and boat conformers.
- Practice drawing the chair conformer, labeling axial and equatorial bonds.
- Prepare to analyze substituent effects on stability in future lessons.