Isomers and Stereochemistry - Organic Chemistry Concepts: An EFL Approach - page 47

FIGURE 3.7
The cis - trans isomers of 1,2-dimethylcyclopropane.
The π bond is a weaker bond, but under normal conditions (about 60-80 kJ/
mol), the bond is strong enough to stop rotation around the bond axis. As a
result, when different substituents are attached to these carbon atoms, cis and
trans isomers are possible. See Figure 3.8 .
FIGURE 3.8
The cis - trans stereoisomers of 2-butene.
The stereoisomers in Figure 3.8 have different stabilities and properties. Under
normal conditions, there is not enough energy for interconversion between this
pair of geometric isomers. However, interconversion is relatively easier, using
heat, catalysis, or photochemical action, because of the lower energy of the π
bond. Note that if the two substituents at one end of a double bond are the
same, then stereoisomers are not possible. See Figure 3.9 .
FIGURE 3.9
Symmetry about the alkene double bond.
The situation is more complex when the alkenes are tri- and tetra-substituted.
The cis / trans naming can no longer be used. Instead, a system of sequence rules
( E/Z system) is used in a specific order to give the name. This system is not
needed at this level, but details can be found in Appendix 5.
3.5 CONFIGURATIONAL ISOMERS
The word configuration describes the exact 3-D arrangement about an sp 3
tetrahedral center. This important type of stereoisomer occurs because carbon
sp 3 tetrahedral centers can allow a molecule to show chirality .