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8/11/2019 Conform Analysis
http://slidepdf.com/reader/full/conform-analysis 1/8
• cyclic systems
strain
cyclohexanes
• A-values
• experimental methods
time constant
Chapter 5. Conformational analysis
Fantastically short version on cyclic systems
8/11/2019 Conform Analysis
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A. Strain
three types: - angle strain: bond angle on sp3 C deviates from 109.5°
- torsion strain: bonds are eclipsed (dihedral less than 60°)
- steric strain: non-bonded repulsion below van der Waals radius
1. Cyclic systems
HH
≡
pseudorotation
≡
puckered
envelope
1,3-diaxial repulsion
8/11/2019 Conform Analysis
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C C
B. Cyclohexanes
- one particularly stable conformation:
1. Cyclic systems continued
HH - “no” angle strain
- “no” torsion strain (ap conformation not possible: )
- some steric strain
H
H
H
H
C C
chair boat twist half-chair
ring inversion coordinate
Epot
[kJ/mol]
H
HH
H
H
H
conformers in equilibrium:
45
2520
0
less than 0.5%
1
1
12 2
2
ring inversion involves all four:
- 4 conformations, 2 conformers:
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B. Cyclohexanes
Calculation of the equilibrium composition:
- from the free energy and the equilibrium constant:
H
HH
H
H
H
conformers in equilibrium:
less than 0.5%
1
1
12 2
2
Δ
G°
= -RT lnK
RT
Go
eK
Δ−
=R = 8.31 J/molK
T = 298 K (RT, often but not always)
ΔG° ≈ ΔE: no large ΔS changes
equatorial
axial
N
N =
1. Cyclic systems continued
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B. Cyclohexanes
I. MonosubstitutionR
R
ΔG° is called “A-value”: energy difference between axial and equatorial
(axial-equatorial, usually positive value)
RΔG° or A-value
[kJ/mol] K (298K)
CH3 7.1
t-Bu 20 (“anchor”)
Br 2.0
(Ph 12)
18
1
100
6
1
06.006.0298/31.8
/1000/1.7
===== ⋅
⋅−
K molK J
kJ J molkJ
e
a e N
N
BrBr
ν(C-Br) 685 cm-1 658 cm-1
±0 kJ/mol+x kJ/mol
1. Cyclic systems continued
or 95%e : 5%a
equatorial >99% (ratio is 3·10-4 : 1)
about 60%e : 40%a, both can be identified in the IR (25 °C):
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B. Cyclohexanes
II. Disubstitution
- symmetric disubstitution
1,2-trans or 1,3-cis or 1,4-trans
a,a e,e a,e e,a
e,e is more stable K = 1
- other disubstitution cases
OH
CH3 OH
CH3
?
A-value[kJ/mol]
OH 3.77 : e is preferred, but only by 4 kJ/mol
CH3 7.1 : e is much more preferredOH
CH3⇒ is dominant
OH CH3
Epot
[kJ/mol]
7.1
3.77
0 e e
a
a
1. Cyclic systems continued
1,2-cis or 1,3-trans or 1,4-cis
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B. Cyclohexanes
II. Special cases
would imply
1. Cyclic systems continued
t-Bu t-Bu
t-Bu
t-Bu, but A-value for t-Bu is ≈ 20 kJ/mol
ΔE(twist-chair) is ≈ 20 kJ/mol
⇒ mixture oft-Bu
t-Buand
t-But-Bu
Et
Et
Et Et
Et
Et
i-Pr
i-Pr
i-Pr i-Pr
i-Pr
i-Pr
EtEt
Et
EtEt
EtH
HH
H
all-e:
all-a! , because too much hindrance in all-e:
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Search for a) geometries of conformers (e or a, planar or not, sc or ap)⇒ static methods
b) energies (relative stabilities of conformers, activation energies)
⇒ dynamic methods
2. Experimental methods
Wavelength
(cm)
Timeconstant
(s)Diffraction:X-ray, neutron,electron
10-8 10-15 fast method, not a dynamic method: snapshots; no Ea
UV/Vis
IR, Raman
10-5 – 10-4
10-4
– 10-2
10-13
10-12
can detect dynamics; no Ea
Microwave 101 – 10-1 10-10 for Ea < 12 kJ/mol: small barriers
NMR 102 10-8 for Ea ≈ 30 - 120 kJ/mol: larger barriers, slow method
For even larger barriers (> 120 kJ/mol): even slower methods: kinetics!
λ ν
chh E ==