Isomers and Stereochemistry - Organic Chemistry Concepts: An EFL Approach - page 44

3.3 CONFORMATIONAL ISOMERS
Conformational isomers , or conformers for short, are caused by the rotation
around covalent single σ bonds and the three-dimensional (3-D) tetrahedral
shape of the sp 3 -hybridized centers. As you will see in Chapter 8, in larger bio-
molecules such as proteins and enzymes, the overall conformational shape of
the molecule can be necessary for its biological activity.
3.3.1 Conformations in Alkanes
Two common types of diagrams are used to show conformers. See Figure 3.2 .
The first is a Sawhorse representation, which is an angled view along the rotating
bond. The second is a Newman projection, which is an end-on view along the
rotation bond with a circle to represent the front carbon center. Bonding to the
front carbon is drawn to the center of the circle. Bonding to the rear carbon are
drawn only to the edge of the circle.
FIGURE 3.2
Extreme conformers of ethane.
Unlike structural isomers, conformers can interconvert easily because the energy
for rotation is small compared to the energy in the system under normal condi-
tions. Still, there are small energy differences between conformers, and some are
more stable (of lower energy) than others.
For example, in ethane, the energy difference between the two extreme con-
formers is about 12 kJ/mol. The lower energy staggered conformer has the C-H
bonds rotated as far apart as possible. The higher energy eclipsed conformer has
the C-H bonds lined up as shown in Figure 3.2 . Because 60-80 kJ is available
at room temperature, interconversion over all the possible conformers between
these two extremes occurs easily.
As seen in Figure 3.3 , rotation around the central C-C bond for butane gives two
different staggered forms and two different eclipsed forms. This is because two of the
hydrogen substituents from the ethane model in Figure 3.2 are now methyl groups.
The simple situations in ethane and butane are multiplied as the molecules
become more complex. In these, the specific rotational energies around each
C-C bond can be different. As Figure 3.4 indicates, the lowest energy conforma-
tion in alkane systems usually tries to have the greatest number of bonds with
staggered arrangement.