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• cyclic systems

strain

cyclohexanes

• A-values

• experimental methods

time constant

Chapter 5. Conformational analysis

Fantastically short version on cyclic systems

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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

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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    ==