Isomers and Stereochemistry - Organic Chemistry Concepts: An EFL Approach - page 46

FIGURE 3.6
Conformational interconversions in cyclohexane.
cyclohexane, these two sets of hydrogens easily exchange between the two alter-
nate chair conformers. However, this is not true when the ring has substituents
that are not just hydrogen.
3.4 GEOMETRIC ( CIS - TRANS ) ISOMERS
Geometric isomers can be caused if a structural reason stops the bond rotation
which would let the isomers convert from one to the other. This is common in
both alkenes, because of the C ] C, and cycloalkanes, because of the ring. Both
of these features can cause substituents to have two different geometric relation-
ships with one another. These relationships are called cis , which means on the
same side, or trans , which means on the opposite side.
As mentioned earlier, we see that the structural isomers have different orders of
their bonded atoms. However, geometric isomers have the same order of atomic
bonding, but different arrangements in space. Any compounds that have the
same order of atomic bonding, but different arrangement in space are called
stereoisomers . Therefore, cis - trans isomers are one type of stereoisomers.
3.4.1 Cis - Trans Isomers in Cycloalkanes
With their cyclic structure, cycloalkanes can be looked at from two sides, a “top”
side and a “bottom” side. Because of this, in substituted cycloalkanes, isomers
become possible when the substituents have different geometric relationships to
one another ( Figure 3.7 ).
Cis - trans isomers are different compounds with different properties. To convert
between these, strong covalent single σ bonds need to be broken and formed.
3.4.2 Cis - Trans Isomers in Alkenes
In compounds with a C ] C, the bonding between the two sp 2 -hybridized car-
bon atoms has a strong σ bond, ∼340 kJ/mol, and a weaker π bond, ∼270 kJ/mol.