Chapter 5: Rings

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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:

R

RR

R

109.5°

planar structrue; C-C bond angles are defined as 60°

cyclopropane

60° bond angles 90°(if planar)

120°(if planar)

150°(if planar)

severe angle strain small angle strain no angle strain

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:

H

HH

HH

H

H

HH

H

H

H

ΔG = 3 Kcal/mol

Each H-H eclipsing interaction is 1 Kcal/mol

6 Kcal/mol 8 Kcal/mol 10 Kcal/mol 12 Kcal/mol

H

HH

H

H

H

planar structrue; C-C bond angles are defined as 60°

cyclopropane

H

H

H

H

H

H

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.

H

H

H

H

pull "tip" up or down

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

H

H

H

H

H

H

H

H

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:

axial positions equatorial positions

ΔG = 0

CH3

CH3

ΔG not 0

Ring Structures

CH3

CH3

ΔG not 0

CH3

H

H

H

H

H

H

H

CH3

H

H

CH3

H

H

H

H

H

CH3

axial substituent has 2 gauche-butane interactions (+1.8 kcal/mol)

equatorial substituents are in anti-butane conforamation (lowest energy)

Recall the butane conformational isomers:

This relates to cycohexane equilibria:

H

CH3

H

HH

CH3

H

CH3

H

CH3H

H

anti-butane gauche-butane(+0.6 kcal/mol)

Ring Structures

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

R

RΔG = ?

R

Me

Et

Pr

iPr

tBu

ΔG (kcal/mol)

1.74

1.78

2.21

2.61

5.5

equatorial:axial ratio (25 °C)

19:1

21:1

43:1

86:1

11,900:1

Ring Structures

Ring size Total Strain(kcal/mol)

Strain per CH2(kcal/mol)

3456789101112131415

27.526.3

6.20.16.29.7

12.612.411.3

4.15.21.91.9

9.176.581.240.020.891.211.401.241.020.340.400.140.13

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