Chemistry Reference
In-Depth Information
(a)
(c)
(d)
(b)
Figure 3.30
The structure of cyclohexane: (a) standard chemical drawing diagram, (b) ball-
and-stick model showing ring puckering in chair conformation and (c) view down C
3
principal
axis.
alternative angles and so it is much easier to pin down the symmetry elements. For exam-
ple, in the cyclohexane case, looking down on to the six ring to give the view shown in
Figure 3.30d reveals that the principal axis is
C
3
, so we are on the right-hand branch of
the flow diagram (Figure 3.29) with
n
= 3. Still looking down the principal axis, it appears
that a
C
6
rotation will bring each carbon atom to the position of one of its neighbours.
However, due to the ring pucker, alternate carbon atoms are actually on opposite sides of
the average C atom plane. This means that a
C
6
rotation followed by a reflection is a sym-
metry operation but a simple
C
6
rotation is not, i.e. we have an
S
6
axis collinear with the
C
3
.This
S
6
operation can also be seen from the positions of the H atoms in the side view of
Figure 3.30c, since the vertical C H bonds alternate from pointing up to pointing down
around the ring. The next question on the flow diagram asks if there are further elements
other than
i
(remember
i
S
2
1
). There are other elements; in particular, there are three
horizontal
C
2
axes which run between opposite bond centres, and so we must have one of
the
D
point groups. There is no horizontal plane of symmetry, but there are vertical planes
which contain opposite CH
2
groups. These bisect the angle between the horizontal
C
2
axes
and so are labelled
=
σ
d
. With a principal axis order of 3 we have the point group
D
3d
.
Another possible conformation for cyclohexane is the boat structure shown in
Figure 3.31. This structure is slightly higher in energy than the chair conformer, but is still
accessible at room temperature and above. In the boat conformation, the ring puckering
(a)
(b)
(c)
Figure 3.31
Cyclohexane in the boat conformation: (a) chemical drawing representations,
(b) side view and (c) view down C
2
axis of ball-and-stick model.
