Isomers and Stereochemistry - Organic Chemistry Concepts: An EFL Approach - page 45

FIGURE 3.3
Staggered and eclipsed conformers for butane.
FIGURE 3.4
Fully extended staggered alkane array.
3.3.2 Conformations in Cycloalkanes
While C 3 -C 5 cycloalkanes are nearly planar, rings of six or more carbons have
enough flexibility for some rotation around the bonds. Because the bonded cen-
ters are sp 3 -hybridized, this leads to conformational changes.
The very common cyclohexane ring shows this feature clearly. There are two
extreme conformations that differ by 23 kJ/mol in energy. The lower energy con-
former is called a chair, and the higher energy conformer is called a boat. See
Figure 3.5 .
FIGURE 3.5
Cyclohexane conformers.
Cyclohexane can easily convert between the two possible lowest energy chair
forms. This conversion goes through a boat, and needs about 45 kJ/mol to occur.
See Figure. 3.6 .
Note that the hydrogen substituents of the chair are divided into two sets of six
each. The set in the overall plane of the ring is called equatorial. The other set,
at right angles to the overall plane of the ring, is called axial. In unsubstituted