Waals repulsion) results when nonadjacent atoms or groups compete for the same space.
Nonbonded strain is the dominant source of steric strain in the flagpole interactions of the
cyclohexane boat conformation. To alleviate the strain, cycloalkanes attempt to adopt various
nonplanar conformations. Cyclobutane puckers into a slight “V” shape; cyclopentane adopts what is
called an envelope conformation; and cyclohexane (the one you will undoubtedly see the most on
the MCAT) exists mainly in three conformations called the chair, boat, and twist- or skew-boat
forms. These cycloalkanes are shown in Figure 2.6.
Figure 2.6. Conformations of CycloalkanesThe most stable conformation of cyclohexane is the chair conformation, which minimizes all three
types of strain. The hydrogen atoms that are perpendicular to the plane of the ring (sticking up or
down) are called axial, and those parallel (sticking out) are called equatorial. The axial–equatorial
orientations alternate around the ring; that is, if the wedge on C-1 is an axial group, the dash on C-2
will also be axial, the wedge on C-3 will be axial, and so on.
Cyclohexane can undergo a chair flip in which one chair form is converted to the other. In this
process, the cyclohexane molecule briefly passes through a fourth conformation called the half-
chair conformation. After the chair flip, all axial groups become equatorial and all equatorial groups
become axial. All dashes remain dashes, and all wedges remain wedges. This interconversion can be
slowed if a bulky group is attached to the ring; tert-butyl groups are classic examples of bulky groups
on the MCAT. For substituted rings, the bulkiest group will favor the equatorial position to reduce
nonbonded strain (flagpole interactions) with axial groups in the molecule, as shown in Figure 2.7.