Conformational Analysis
An important aspect of organic compounds is that the compound is not static,
but rather has conformational freedom by rotating, stretching and bending about bonds
Each different arrangement in
...
Conformational Analysis
An important aspect of organic compounds is that the compound is not static,
but rather has conformational freedom by rotating, stretching and bending about bonds
Each different arrangement in space of the atoms is called a “Conformer”
(a less used term is a “Rotamer” if change is caused by a bond rotation)
Different conformers can have vastly different energies and the relative proportion
of each conformer is related to the energy difference between them
CH3
H3C H3C CH3
CH3
H H
CH3
H H
H
H CH3
CH3
H H
To observe the spatial
arrangement of atoms
in space, often draw
conformers in a
Newman projection
Lower Energy
196$
Conformational Analysis
Conformers will be of different energy due to strain
Sources of strain are generally categorized in one of three types:
1) Torsional strain
E
n
e
r
g
y
torsional angle
-60˚ 0˚ 60˚ 120˚ 180˚ 240˚
Torsional strain is due to interactions as groups change relative position
with a change in torsional bond angle
Consider rotation of ethane
H
H
H
H
H
H
H
H
H
H H
H
Staggered
conformation
Eclipsed
conformation
Eclipsed is higher in
energy than staggered
due to increased
torsional strain
Further rotation will
convert eclipsed
conformation back to
staggered
H
H
H
H
H
H
~3 kcal/mol
197$
Conformational Analysis
2) van der Waals strain
Another source of strain is when groups are placed in positions closer
than the sum of their van der Waals radii
Consider rotation of butane
E
n
e
r
g
y
torsional angle
-60˚ 0˚ 60˚ 120˚ 180˚ 240˚
CH3
H H
CH3
H H
anti
H
H
CH3
CH3
H H
H
H CH3
CH3
H H
H
H3C
H
CH3
H H
gauche
totally
eclipsed eclipsed
“gauche” conformation is
higher in energy than anti
(both are “staggered”
conformations)
“totally eclipsed”
conformation (which has
largest groups eclipsing each
other) is higher in energy than
other eclipsed conformations
198$
Conformational Analysis
The difference in energy thus affects the amount of each conformer pr
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