Chemistry Reference
In-Depth Information
the sample, the energy available to overcome the
interconversion energy barrier diminishes, until at
a sufficiently low temperature, the interconversion
stops, and two types of hydrogen are detectable in
the
not planar, but has a chair conformation. We shall fre-
quently want to use the hexagon representation, and
it will be necessary to assign hydrogens or other sub-
stituents onto the chair representation with the correct
stereochemistry. At this stage it is salutary to look at
both the two-dimensional hexagon and the chair rep-
resentations of cyclohexane. Note particularly that we
must not confuse 'up' with axial, and 'down' with
equatorial. As the structures show, 'up' hydrogens or
substituents will alternate axial and equatorial as we
go round the ring positions.
89
◦
C.
Measurement of this temperature allows the energy
barrier to be calculated.
If we look at the two-dimensional hexagon repre-
sentation for cyclohexane, we could put in the bonds
to hydrogens as wedges (up bonds) or dotted lines
(down bonds). We now know the cyclohexane ring is
NMR spectrum.
This
temperature is
−
up
up
H
up
H
up
H
H
H
H
H
H
H
H
hydrogens shown in bold are 'up'
and alternate axial
H
H
down
H
H
H
H
H
H
equatorial
around the ring; they are not all
axial or all equatorial
−
up
down
H
H
H
H
H
H
H
H
up
H
H
H
H
down
H
H
H
down
H
H
H
down
down
incorrect planar
representation
The chair is not the only conformation that
cyclohexane might adopt. An alternative
boat
conformation
is attained if the ring flip-type process
is confined to just one carbon. The name boat
comes from the similarity to boats formed by paper
folding; sea-worthiness is rather questionable. Again,
there
is
no
ring
strain
in
this
conformation,
but
it
turns
out
that
some
of
the
C-H
bonds
are
eclipsed,
as
seen
in
the
accompanying
Newman
projection.
flagpole interaction
H
• no ring strain
• eclipsing in
boat
conformation
• flagpole interaction in boat
• eclipsing and flagpole interaction
reduced in
twist-boat
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
• both higher energy than chair
H
H
H
H
chair
boat
twist-boat
CH
2
H
H
CH
2
H
H
H
H
H
H
Newman projection of boat
conformation looking along
the two horizontal bonds;
some bonds are eclipsed
