CONFORMATIONAL ISOMERS
57
Stereoisomers, on the other hand, are compounds
with the same molecular formula, and the same
sequence of covalently bonded atoms, but with
a different spatial orientation. Two major classes
of stereoisomers are recognized, conformational
isomers and configurational isomers.
Conformational isomers, or conformers, interconvert easily by rotation about single bonds. Configurational isomers interconvert only with difficulty and,
if they do, usually require bond breaking. We shall
study these in turn.
3.3 Conformational isomers
3.3.1 Conformations of acyclic compounds
Let us consider first the simple alkane ethane. Since
both carbons have a tetrahedral array of bonds,
ethane may be drawn in the form of a wedge–dot
representation.
Now let us consider rotation of the right-hand
methyl group about the C–C bond, and we eventually
get to a different wedge–dot representation as shown.
This is more easily visualized by looking at the
molecule from one end down the C–C bond, and
this gives us what is termed a Newman projection.
The Newman projection shows the hydrogen atoms
and their bonds, but the carbons are represented by
a circle; since we are looking down the C–C bond,
we cannot see the rear carbon. A further feature
is that the C–H bonds of the methyl closest to
us are shown drawn to the centre of this circle,
whilst those of the rear methyl are partially obscured
and drawn only to the edge of this circle. We can
draw a similar Newman projection for the second
wedge–dot representation, but the C–H bonds of the
front and rear methyls will appear to be on top of each
other. We therefore draw a slightly modified version
showing all bonds, but must remember that this really
represents a system where the bonds at the rear are
obscured by the bonds at the front.
In the sawhorse representation, the molecule is
viewed from an oblique angle, and all bonds can be
seen.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
sawhorse
representations
H
H
H
H
H
H
The two representations shown here are actually
two different conformers of ethane; there will be
an infinite number of such conformers, depending
upon the amount of rotation about the C–C bond.
Although there is fairly free rotation about this
bond, there does exist a small energy barrier to
rotation of about 12 kJ mol
−1 due to repulsion of
the electrons in the C–H bonds. By inspecting
the Newman projections, it can be predicted that
this repulsion will be a minimum when the C–H
bonds are positioned as far away from each other
wedge–dot
representation
view from end gives Newman projection
staggered conformer
low energy
eclipsed conformer
high energy
view
from end
C C
H
H
H
H
H
H
C C
H
H
H
H
H
H
rotation of right
hand methyl
generally
drawn as
shows both bonds,
but angle is
assumed to be 0˚
60˚
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C C bond
about
57
Stereoisomers, on the other hand, are compounds
with the same molecular formula, and the same
sequence of covalently bonded atoms, but with
a different spatial orientation. Two major classes
of stereoisomers are recognized, conformational
isomers and configurational isomers.
Conformational isomers, or conformers, interconvert easily by rotation about single bonds. Configurational isomers interconvert only with difficulty and,
if they do, usually require bond breaking. We shall
study these in turn.
3.3 Conformational isomers
3.3.1 Conformations of acyclic compounds
Let us consider first the simple alkane ethane. Since
both carbons have a tetrahedral array of bonds,
ethane may be drawn in the form of a wedge–dot
representation.
Now let us consider rotation of the right-hand
methyl group about the C–C bond, and we eventually
get to a different wedge–dot representation as shown.
This is more easily visualized by looking at the
molecule from one end down the C–C bond, and
this gives us what is termed a Newman projection.
The Newman projection shows the hydrogen atoms
and their bonds, but the carbons are represented by
a circle; since we are looking down the C–C bond,
we cannot see the rear carbon. A further feature
is that the C–H bonds of the methyl closest to
us are shown drawn to the centre of this circle,
whilst those of the rear methyl are partially obscured
and drawn only to the edge of this circle. We can
draw a similar Newman projection for the second
wedge–dot representation, but the C–H bonds of the
front and rear methyls will appear to be on top of each
other. We therefore draw a slightly modified version
showing all bonds, but must remember that this really
represents a system where the bonds at the rear are
obscured by the bonds at the front.
In the sawhorse representation, the molecule is
viewed from an oblique angle, and all bonds can be
seen.
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
sawhorse
representations
H
H
H
H
H
H
The two representations shown here are actually
two different conformers of ethane; there will be
an infinite number of such conformers, depending
upon the amount of rotation about the C–C bond.
Although there is fairly free rotation about this
bond, there does exist a small energy barrier to
rotation of about 12 kJ mol
−1 due to repulsion of
the electrons in the C–H bonds. By inspecting
the Newman projections, it can be predicted that
this repulsion will be a minimum when the C–H
bonds are positioned as far away from each other
wedge–dot
representation
view from end gives Newman projection
staggered conformer
low energy
eclipsed conformer
high energy
view
from end
C C
H
H
H
H
H
H
C C
H
H
H
H
H
H
rotation of right
hand methyl
generally
drawn as
shows both bonds,
but angle is
assumed to be 0˚
60˚
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C C bond
about