Overview
This lecture covers the structure, stability, and conformations of cycloalkanes, focusing on cyclohexane and its chair conformations, including their drawing conventions, stability, and analysis of substituted cyclohexanes.
Introduction to Cycloalkanes and Ring Strain
- Cycloalkanes are alkanes forming ring structures; common examples are cyclopropane, cyclobutane, cyclopentane, cyclohexane, and cycloheptane.
- Cyclopropane is highly strained due to very small 60° bond angles (angle strain), leading to instability.
- Ring strain consists of angle strain, torsional strain (from eclipsed bonds), and steric strain (from atoms crowding).
- Smaller rings like cyclopropane and cyclobutane have more ring strain; cyclohexane in the chair form has almost none.
Cyclohexane Conformations
- Cyclohexane adopts several conformations: chair (most stable), boat, and twist-boat (higher energy, less common).
- Chair conformations are the focus due to their stability and prevalence in nature.
- The chair conformations interconvert via a "chair flip," exchanging the positions of axial and equatorial bonds.
Axial and Equatorial Positions
- Each carbon in cyclohexane has two bonds: one axial (vertical, up or down) and one equatorial (around the ring's "equator," slanted out).
- Axial positions alternate up and down around the ring.
- Equatorial positions alternate in direction and are opposite to the axial on each carbon; they are slanted left or right depending on position.
Substituent Effects and Stability
- Substituents prefer equatorial over axial positions to minimize 1,3-diaxial interactions (steric hindrance with axial hydrogens on carbons 1 and 3).
- Larger groups (e.g., t-butyl) strongly prefer the equatorial position; methyl < chlorine < t-butyl in preference.
- The most stable chair conformation has the biggest substituent equatorial, and as many groups equatorial as possible.
Drawing Chair Conformations
- Practice drawing chair conformations, assigning substituents to correct axial/equatorial positions.
- A chair flip swaps all axial positions with equatorial and vice versa.
- Numbering the ring helps assign substituents correctly.
Cis and Trans Isomerism in Cyclohexane
- "Cis" means two groups are on the same side (both up or both down); "trans" means on opposite sides (one up, one down).
- Cis/trans is determined by overall "up" or "down" orientation, not axial/equatorial status.
Newman Projections and Cyclohexane
- Double Newman projections can be used to analyze cyclohexane conformation by looking down two adjacent bonds.
- Positions that are straight up or down are axial; slanted are equatorial.
Key Terms & Definitions
- Cycloalkane — ring-shaped alkane hydrocarbon.
- Angle Strain — instability from bond angles deviating from ideal sp³ (109.5°).
- Torsional Strain — electron repulsion in eclipsed bonds.
- Steric Strain — atom crowding between non-bonded atoms.
- Ring Strain — sum of angle, torsional, and steric strain.
- Chair Conformation — most stable 3D shape of cyclohexane with minimal ring strain.
- Axial Position — bond projecting vertically (up or down) from the ring.
- Equatorial Position — bond projecting outward along the ring’s equator.
- 1,3-Diaxial Interaction — steric hindrance between axial groups on carbons 1 and 3.
- Cis/Trans — relative positions of substituents on rings (cis = same side, trans = opposite sides).
Action Items / Next Steps
- Practice drawing cyclohexane chair conformations and assigning substituents to axial/equatorial positions.
- Review and identify the most stable conformation for disubstituted and trisubstituted cyclohexanes.
- Prepare to recognize and draw cis and trans isomers on chair structures.
- Recommended: Build physical or model-kit chair conformations for visualization.