CONFORMATIONAL ISOMERS
65
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 NMR spectrum. This temperature is −89
◦ C.
Measurement of this temperature allows the energy
barrier to be calculated.
If we look at the two-dimensional hexagon representation 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
not planar, but has a chair conformation. We shall frequently want to use the hexagon representation, and
it will be necessary to assign hydrogens or other substituents 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 representations 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.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
hydrogens shown in bold are 'up'
and alternate axial−equatorial
around the ring; they are not all
axial or all equatorial
H
H
H
H
H
H
H
H
H
H
H
H
up
down
up
up
up
up
up
down
down
down
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.
CH 2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
• no ring strain
• eclipsing in boat conformation
• flagpole interaction in boat
• eclipsing and flagpole interaction
reduced in twist-boat
• both higher energy than chair
boat
twist-boat
chair
H
H
Newman projection of boat
conformation looking along
the two horizontal bonds;
some bonds are eclipsed
H
H
H
H
H
H
H
H
CH 2
flagpole interaction
65
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 NMR spectrum. This temperature is −89
◦ C.
Measurement of this temperature allows the energy
barrier to be calculated.
If we look at the two-dimensional hexagon representation 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
not planar, but has a chair conformation. We shall frequently want to use the hexagon representation, and
it will be necessary to assign hydrogens or other substituents 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 representations 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.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
hydrogens shown in bold are 'up'
and alternate axial−equatorial
around the ring; they are not all
axial or all equatorial
H
H
H
H
H
H
H
H
H
H
H
H
up
down
up
up
up
up
up
down
down
down
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.
CH 2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
• no ring strain
• eclipsing in boat conformation
• flagpole interaction in boat
• eclipsing and flagpole interaction
reduced in twist-boat
• both higher energy than chair
boat
twist-boat
chair
H
H
Newman projection of boat
conformation looking along
the two horizontal bonds;
some bonds are eclipsed
H
H
H
H
H
H
H
H
CH 2
flagpole interaction