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Chapter 5: Rings

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 .

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Chapter 5: Rings

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  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. 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

  10. 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

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