Chemistry Reference
In-Depth Information
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 conforma-
tion
. 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.
Let us look at a simple example, namely
methyl-
cyclohexane
.
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 cyclo-
hexane). It turns out that the conformer with the
equatorial methyl group is favoured over the con-
former 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 hydro-
carbon butane.
Substituted cyclohexanes
The ring flipping conformational mobility in the un-
substituted compound cyclohexane has little practical
significance; but, when the ring is substituted, we
have to take ring flip into account, because one partic-
ular conformation is usually favoured over the other.
methylcyclohexane
interconversion of conformers via ring flip
changes axial methyl to equatorial methyl;
equatorial conformer favoured
Newman projection
down 2,1-bond
Newman projection
down 2,1-bond
CH
3
H
CH
3
H
6
4
H
CH
2
5
H
3
C
CH
2
H
1
5
3
6
6
H
1
H
3
C
2
H
CH
2
H
CH
2
3
3
H
3
2
4
6
H
H
gauche
anti
methyl axial
higher energy conformer
1,2-
gauche
interaction and
1,3-diaxial interactions
methyl equatorial
lower energy conformer
1,2-
anti
and
no 1,3-diaxial interaction
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
