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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