Chemistry 2500
Conformations of Organic Molecules – Fall 2009
Conformations of Organic Molecules
A conformation is an instantaneous orientation of atoms.
 Different conformations arise from bond rotation, NOT
from bond cleavage.
H
H
H
C
H
=
C
H
H
H
H
HH
H
H
C
H
H
H
=
C
H
Sawhorse
H
H
H
H
H
H
H
H
Newman Projection
Conformers – Ethane
eclipse
d
staggere
d
HH
H
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
E
H
H
H
H
H
60
H
120
180
 Dihedral or Torsional Angle

Which conformer is favoured?
240
Conformations - Butane

Build a model of butane. Looking down the C2-C3 bond, draw the
Newman Projections of the four different conformers of butane.

What are the relative energies of these conformers?
Conformers - Cyclohexane

Cyclohexane is the only ring under 14 carbon atoms that is
relatively free from strain. The two most common conformations
of cyclohexane are the chair conformation and the boat
conformation.
H
H
H
C
H
H
H
H
C
C
C
C
H
H
H
C
H H
C
C
H
chair
C
C
H
H
H
H
C
C
H
H
H
H
boat
H
H
Conformers - Cyclohexane



What are the bond angles around each carbon atom in the chair
conformation?
Look down any carbon-carbon bond. What are the relative
positions of adjacent hydrogen atoms?
Notice that the hydrogen atoms are found in one of two positions:
H
H
H
H
C
H
H
C
H
H
C
C
C
H
C
H
H
H
H
H
H H
H
H
H
H
Conformers - Cyclohexane

There are two different chair conformations that interconvert
through a process called a ring flip.
H
H
H
H H
C
H
H

H
H
C
H
H
H
H
H
C
H
H
H
C
H
H
H
What are the consequences of a ring flip?
H
H
H
H
Conformers – Substituted Cyclohexane
What happens when we substitute one of the hydrogen atoms for
another group, say methyl?

H
CH 3
H
H H
H
H
H
H
H
H
H
CH 3
H
H
H
H
H
H
H
H
H
H
CH 3
H
H
H
H
H
H
H
H
H
H
CH 3
H

H
H
Are these two conformations equivalent?
H
H
Conformers – Substituted Cyclohexane
H
H
H
H
X
H
H
H
HH
H
H
H
H
H
H
[Xeq]
K=
H
X
H
H
H Xax.
H
[Xax]
ΔG° = -RTlnK
H H Xeq.
X
K
ΔG° (KJ/mol)
-H
1
0
-CH3
19
-7.3
-CH2CH3
21
-7.5
-C(CH3)3
3300
-20
-F
1.7
-1.3
-Cl
3
-2.7
-Br
3
-2.7
-I
2.3
-2.1
Conformers – Substituted Cyclohexane

Rationalize the relative ΔG° values for the following pairs of
groups:
◦ -CH3 vs -C(CH3)3
◦ -CH3 vs -Cl
◦ -Cl vs -F
◦ -Cl vs -I
Conformers – Disubstituted Cyclohexane

Consider the two chair conformers for cis-1,2-dimethylcyclohexane
and trans-1,2-dimethylcyclohexane
CH 3
CH 3
CH 3
CH 3 H
H
H
H
CH 3
H
H
CH 3

CH 3
H
Is one conformer favoured over the other?
CH 3
H
Conformers – Disubstituted Cyclohexane

Now consider the two chair conformers for cis-1,3dimethylcyclohexane and trans-1,3-dimethylcyclohexane.
CH 3
CH3
H3 C
CH 3
H
H
H
H
CH 3
H
H
CH 3
H
CH 3
CH3

Is one conformer favoured over the other?
H
Conformers – Disubstituted Cyclohexane

What is the most stable chair conformer for cis-1-tbutyl-4methylcyclohexane.
CH 3
CH3
H3 C
C
CH 3
H 3C
CH 3
H
C
H
CH 3
H 3C
H
H

What is the most stable conformer for menthol?
OH
Conformers – Cycloalkanes and Chirallity

Remember, ring flips are like bond rotations – they only change the
conformation of the molecule. Therefore, you are allowed to do
ring flips to determine if your molecule is chiral or not.

Convince yourself that cis-1,2-dichlorocyclohexane is achiral.

Convince yourself that trans-1,2-dichlorocyclohexane is chiral.
Conformers – Smaller Cycloalkanes

Six-membered rings are prevalent in nature because they are
relatively free from:
◦ torsional strain: eclipsed bonds
and
◦ angle strain: bonds angles that deviate from those predicted by VSEPR
theory

Build cyclopentane, cyclobutane, and cyclopropane. Is there
torsional strain? Angle strain? What do you think would be the
most stable conformer for each of these rings? Use the amount of
strain to rank these rings from most to least stable.