1. Chapter 5: Rings

2. Ring Structures
Remember that sp3 carbon wants to be tetrahderal with 109.5° bond angles:
When confined to a ring, bond angles are forced to change:
Bond angles that deviate from the ideal (acyclic) angles increase the energy of the system through angle strain .

3. Torsional Strain
Cyclopropane is planar because it has to be! All other cycloalkanes would have severe torsional strain if they remained planar (in addition to angle strain):
If planar, each of the following cycloalkanes would have a lot of torsional strain:

4. Torsional Strain
To relieve torsional strain (and sometimes angle strain too!), rings larger than 3 atoms will pucker (bend out of planarity):
Both 4 and 5 membered rings are "fluxional" - the bulge moves rapidly around the ring.

5. Chair Conformation of Cyclohexane
Ring Structures
Cyclohexane commonly puckers into a conformer called a chair. This conformation has 109.5° bond angles and a staggered arrangement for all atoms
Chair Conformation of Cyclohexane
If you squint enough, this roughly resembles a beach chair:

6. Ring Structures
Substituents on cyclohexane have 2 different environments:
However, chair-chair interconversion will equilibrate the positions:
Substitution of a methyl makes the 2 chair forms non-equivalent:

7. Ring Structures Recall the butane conformational isomers:
This relates to cycohexane equilibria:

8. Ring Structures
The third kind of ring strain is steric strain ("bumping").

9. Ring Structures
Total strain is the sum of: angle strain, eclipsing interactions (torsional strain), van der Waals (steric strain) interactions