Chapter 5: Rings

Ring Structures Remember that sp 3 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:

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. Torsional Strain

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 If you squint enough, this roughly resembles a beach chair: Chair Conformation of Cyclohexane

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

Multicyclic Ring Structures Adamantium! Why it’s so cool: One of the strongest substances in the universe Why it’s not so cool: Doesn’t exist