Chapter 5: Rings

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 .

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:

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.

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:

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:

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

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

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