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Conformations are the structures that result from rotation around C-C sigma bonds.
Conformations are also called rotamers in some texts.
Conformational analysis is the study of the energy changes that accompany these rotations.
A molecule of ethane is the natural starting point for this discussion since there is only
one C-C sigma bond.
There are an infinite number of conformations for ethane but we only consider two major ones:
The conformation in which all atoms are staggered is the most stable.
The least stable conformation is the eclipsed conformation.
staggered
eclipsed
60° rotation
of C-C bond
The energy barrier to rotation is called a torsional barrier
Torsional strain results from increased repulsions between electron groups (or
between atoms) when these groups are in the eclipsed positions. For ethane,
these groups (atoms) are the hydrogen atoms on each carbon.
At room temp, most molecules have enough energy to surmount these barriers.
There are two kinds of formulas used to illustrate the different conformations of a molecule.
One is called a Newman Projection which is interpreted to mean that we are looking down the
C-C bond (the bond that is rotating) with one carbon atom in front of the other. For ethane:
H
H
H
staggered
H
front carbon remains
stationary, back
carbon is rotated 60°
H
HH
eclipsed
H
H
HH
H
The other type of formula is called a sawhorse projection or 3-D formula. In these formulae,
the carbon backbone is shown and dashes and wedges may be used for 3-D effects. The
atoms may be omitted but are sometimes added for clarity. For ethane:
staggered
or
front carbon remains
stationary, back
carbon is rotated 60°
eclipsed
or
A subtle point: If you are holding a molecular model in hand, you can rotate the C-C bonds
and generate as many conformations as you desire. When we say there are an infinite
number of conformations, we are suspending the rules of quantization. According to quantum
theory, there is a fixed number albiet, a large number, of conformations available to a
molecule; each one energetically accessible by the absorption of the appropriate amount of
energy. Therefore, it is not unreasonable to consider only those conformations in which the
energy change is significant, i.e., only the fully eclipsed and fully staggered conformations.
A word about notation: Consider the eclipsed conformation of ethane. The bond connectivity
between the eclipsing hydrogen atoms is H-C-C-H and this can be illustrated using the
convention shown here:
HH
This notation
represents a H-H
eclipsing interaction
H
H
H
H
HH
Eclipsing interactions are also shown as arcs between atoms on the ball-and-stick figures:
H-H eclipsing interactions
shown using arcs
between groups
Conformational analysis for ethane, propane and butane is shown below.
Ethane
Conformer
relative energy (kcal)
interactions
staggered
zero
fully staggered
eclipsed
3.0 kcal
3 eclipsing interactions:
H-H H-H H-H
1.0 1.0 1.0 ~ 3.0 kcal
staggered
end-to-end view
eclipsed
end-to-end view
H-H eclipsing interaction
3 H-H eclipsing
interactions =
approx. 3 kcal
The eclipsed conformation of ethane is 3.0 kcal higher in energy than the
staggered conformation.
Propane
Conformer
relative energy (kcal)
interactions
staggered
zero
fully staggered
eclipsed
3.4 kcal
eclipsing interactions:
H-H H-H H-CH3
1.0 1.0 1.4 ~ 3.4 kcal
staggered
eclipsed
end-to-end view
end-to-end view
CH3 - H eclipsing interaction
two H-H interactions = 2.0 kcal
CH3 - H interaction = 1.4 kcal
Total relative energy = 3.4 kcal
The eclipsed conformation of propane is 3.4 kcal higher in energy than the
staggered conformation.
Butane
Butane has two staggered conformations and two eclipsed conformations.
Conformer
relative energy (kcal)
interactions
anti (staggered)
0
fully staggered,
CH3 groups farthest apart (anti)
eclipsed conformer
3.8
eclipsing interactions:
H-H H-CH3 H-CH3
1.0
1.4 1.4 ~ 3.8 kcal
gauche (staggered)
0.9
CH3-CH3 gauche interaction
0.9 kcal
syn (eclipsed)
4.5
fully eclipsed,
CH3 groups closest (syn)
eclipsing interactions:
H-H H-H CH3-CH3
1.0 1.0 2.5 ~ 4.5 kcal
rotate clockwise around C2-C3
bond to generate conformers
view conformers by looking down
the C(2) - C(3) sigma bond
flip molecule to position
the C(2)-C(3) bond
perpendicular to page
Summary of energy values for specific interactions:
H-H eclipsing interaction is approx. 1 kcal
H-CH3 eclipsing interaction is approx. 1.4 kcal
CH3-CH3 eclipsing interaction is approx. 2.4 kcal
CH3-CH3 gauche interaction is approx. 0.9 kcal
Major conformations for butane are shown below.
Anti conformer (staggered)
end-to-end views- looking down the C(2)-C(3) bond
fully staggered
relative
energy
= zero
eclipsed conformer
CH3 H
CH3 H
H H
CH3 H
1.4
CH3 H
1.4
H
1.0
H
3.8 kcal
gauche conformer (staggered)
CH3
CH3
gauche
interaction
CH3
CH3
0.9 kcal
syn conformer (eclipsed)
CH3 CH3 2.5
CH3 CH3
H
H
1.0
H
H
1.0
4.5 kcal