CONFORMATIONAL ISOMERS
67
In cyclohexene, the double bond and adjacent
carbons must all be planar. The remainder of the
molecule avoids unfavourable eclipsing interactions
by adopting what is termed a half-chair conformation. This would also be found in a cyclohexane ring
fused onto an aromatic ring (tetrahydronaphthalene)
or fused to a three-membered ring (see Section 3.5.2).
The half-chair conformation in cyclohexane (without
the double bond) is thought to be equivalent to the
energy maximum in Figure 3.3 that must be overcome
in the chair–twist-boat interconversion.
Substituted cyclohexanes
The ring flipping conformational mobility in the unsubstituted compound cyclohexane has little practical
significance; but, when the ring is substituted, we
have to take ring flip into account, because one particular conformation is usually favoured over the other.
Let us look at a simple example, namely methylcyclohexane.
Ring flip in the case of methylcyclohexane
achieves interconversion of one conformer where the
methyl group is equatorial into a conformer where
this group is axial (compare the hydrogens in cyclohexane). It turns out that the conformer with the
equatorial methyl group is favoured over the conformer where the methyl group is axial. The energy
difference of these two conformers is estimated to
be about 7.1 kJ mol
−1 ; this is the energy difference,
not the barrier to interconversion. Because of this
energy difference, the equilibrium mixture at room
temperature has about 95% of conformers with the
equatorial methyl and only 5% where the methyl is
axial. We can account for the difference in energy
between the two conformers quite easily using the
reasoning we applied earlier for the acyclic hydrocarbon butane.
H 3 C
H
CH 3
H
H
methylcyclohexane
methyl equatorial
lower energy conformer
1,2-anti and
no 1,3-diaxial interaction
interconversion of conformers via ring flip
changes axial methyl to equatorial methyl;
equatorial conformer favoured
methyl axial
higher energy conformer
1,2-gauche interaction and
1,3-diaxial interactions
gauche
anti
1
2
3
Newman projection
down 2,1-bond
Newman projection
down 2,1-bond
1
2
3
6
6
5
4
5
4
H
CH 3
H
CH 2
CH 2
H
H
CH 2
H 3 C
H
CH 2
H
H
6
3
3
6
We need to consider a Newman projection looking
down the 2,1 bond. When the methyl is axial, it
can be seen that there will be a gauche interaction
between this methyl and the ring methylene (C-3); a
second, similar interaction will be seen if we looked
down the 6,1 bond. Now, in the conformer where
the methyl is equatorial the Newman projection
shows the most favourable anti arrangement for the
methyl and methylene(s); there will be a similar anti
interaction if we looked down the 6,1 bond. On
67
In cyclohexene, the double bond and adjacent
carbons must all be planar. The remainder of the
molecule avoids unfavourable eclipsing interactions
by adopting what is termed a half-chair conformation. This would also be found in a cyclohexane ring
fused onto an aromatic ring (tetrahydronaphthalene)
or fused to a three-membered ring (see Section 3.5.2).
The half-chair conformation in cyclohexane (without
the double bond) is thought to be equivalent to the
energy maximum in Figure 3.3 that must be overcome
in the chair–twist-boat interconversion.
Substituted cyclohexanes
The ring flipping conformational mobility in the unsubstituted compound cyclohexane has little practical
significance; but, when the ring is substituted, we
have to take ring flip into account, because one particular conformation is usually favoured over the other.
Let us look at a simple example, namely methylcyclohexane.
Ring flip in the case of methylcyclohexane
achieves interconversion of one conformer where the
methyl group is equatorial into a conformer where
this group is axial (compare the hydrogens in cyclohexane). It turns out that the conformer with the
equatorial methyl group is favoured over the conformer where the methyl group is axial. The energy
difference of these two conformers is estimated to
be about 7.1 kJ mol
−1 ; this is the energy difference,
not the barrier to interconversion. Because of this
energy difference, the equilibrium mixture at room
temperature has about 95% of conformers with the
equatorial methyl and only 5% where the methyl is
axial. We can account for the difference in energy
between the two conformers quite easily using the
reasoning we applied earlier for the acyclic hydrocarbon butane.
H 3 C
H
CH 3
H
H
methylcyclohexane
methyl equatorial
lower energy conformer
1,2-anti and
no 1,3-diaxial interaction
interconversion of conformers via ring flip
changes axial methyl to equatorial methyl;
equatorial conformer favoured
methyl axial
higher energy conformer
1,2-gauche interaction and
1,3-diaxial interactions
gauche
anti
1
2
3
Newman projection
down 2,1-bond
Newman projection
down 2,1-bond
1
2
3
6
6
5
4
5
4
H
CH 3
H
CH 2
CH 2
H
H
CH 2
H 3 C
H
CH 2
H
H
6
3
3
6
We need to consider a Newman projection looking
down the 2,1 bond. When the methyl is axial, it
can be seen that there will be a gauche interaction
between this methyl and the ring methylene (C-3); a
second, similar interaction will be seen if we looked
down the 6,1 bond. Now, in the conformer where
the methyl is equatorial the Newman projection
shows the most favourable anti arrangement for the
methyl and methylene(s); there will be a similar anti
interaction if we looked down the 6,1 bond. On