Again, in reality, cyclohexane is not a planar molecule. To strike a
balance between torsional strain and angle strain, and to achieve more
stability, cyclohexane attains various conformations, among which the chair
and boat conformations are most significant. At any one moment 99.9 per
cent of cyclohexane molecules will have the chair conformation.
The chair conformation of cyclohexane is the most stable conformer. In the
chair conformation, the C–C–C angles can reach the strain free tetrahedral
value (109.5
), and all neighbouring C–H bonds are staggered. Therefore, this
conformation does not have any angle strain or torsional strain.
Another conformation of cyclohexane is the boat conformation. Here the
H atoms on C 2 –C 3 and C 5 –C 6 are eclipsed, which results in an increased
torsional strain. Also, the H atoms on C 1 and C 4 are close enough to produce
steric strain.
In the chair conformation of cyclohexane, there are two types of position
for the substituents on the ring, axial (perpendicular to the ring, i.e. parallel
to the ring axis) and equatorial (in the plane of the ring, i.e. around the ring
equator) positions. Six hydrogen atoms are in the axial positions and six
others in the equatorial positions. Each carbon atom in the cyclohexane chair
conformation has an axial hydrogen and an equatorial hydrogen atom, and
each side of the ring has three axial and three equatorial hydrogen atoms.
H
H
Chair conformation of cyclohexane
Six axial (a) and six equatorial (e) hydrogen atoms
Chair conformation of cyclohexane
Diaxial interaction
H
H
H
H
H
H
H
H
H
H
a
a
a
a
a
a
e
e
e
e
e
e
H
H
H
H
H
H
H
H
H
H
H
H
a
a
a
a
a
a
e
e
e
e
e
e
When all 12 substituents are hydrogen atoms, there is no steric strain. The
presence of any groups larger than H changes the stability by increasing the
steric strain, especially if these groups are present in axial positions. When
axial, diaxial interaction can cause steric strain. In the equatorial case, there
is more room and less steric strain. Bulky groups always preferably occupy
equatorial positions.
Because of axial and equatorial positions in the chair conformation of
cyclohexane, one might expect to see two isomeric forms of a monosubstituted cyclohexane. However, in reality, only one monosubstituted form
exists, because cyclohexane rings are conformationally mobile at room
temperature. Different chair conformations interconvert, resulting in the
exchange of axial and equatorial positions. This interconversion of chair
conformations is known as a ring-flip. During ring-flip, the middle four carbon
atoms remain in place, while the two ends are folded in opposite directions.
As a result, an axial substituent in one chair form of cyclohexane becomes an
equatorial substituent in the ring-flipped chair form, and vice versa.
3.2 ISOMERISM
41
balance between torsional strain and angle strain, and to achieve more
stability, cyclohexane attains various conformations, among which the chair
and boat conformations are most significant. At any one moment 99.9 per
cent of cyclohexane molecules will have the chair conformation.
The chair conformation of cyclohexane is the most stable conformer. In the
chair conformation, the C–C–C angles can reach the strain free tetrahedral
value (109.5
), and all neighbouring C–H bonds are staggered. Therefore, this
conformation does not have any angle strain or torsional strain.
Another conformation of cyclohexane is the boat conformation. Here the
H atoms on C 2 –C 3 and C 5 –C 6 are eclipsed, which results in an increased
torsional strain. Also, the H atoms on C 1 and C 4 are close enough to produce
steric strain.
In the chair conformation of cyclohexane, there are two types of position
for the substituents on the ring, axial (perpendicular to the ring, i.e. parallel
to the ring axis) and equatorial (in the plane of the ring, i.e. around the ring
equator) positions. Six hydrogen atoms are in the axial positions and six
others in the equatorial positions. Each carbon atom in the cyclohexane chair
conformation has an axial hydrogen and an equatorial hydrogen atom, and
each side of the ring has three axial and three equatorial hydrogen atoms.
H
H
Chair conformation of cyclohexane
Six axial (a) and six equatorial (e) hydrogen atoms
Chair conformation of cyclohexane
Diaxial interaction
H
H
H
H
H
H
H
H
H
H
a
a
a
a
a
a
e
e
e
e
e
e
H
H
H
H
H
H
H
H
H
H
H
H
a
a
a
a
a
a
e
e
e
e
e
e
When all 12 substituents are hydrogen atoms, there is no steric strain. The
presence of any groups larger than H changes the stability by increasing the
steric strain, especially if these groups are present in axial positions. When
axial, diaxial interaction can cause steric strain. In the equatorial case, there
is more room and less steric strain. Bulky groups always preferably occupy
equatorial positions.
Because of axial and equatorial positions in the chair conformation of
cyclohexane, one might expect to see two isomeric forms of a monosubstituted cyclohexane. However, in reality, only one monosubstituted form
exists, because cyclohexane rings are conformationally mobile at room
temperature. Different chair conformations interconvert, resulting in the
exchange of axial and equatorial positions. This interconversion of chair
conformations is known as a ring-flip. During ring-flip, the middle four carbon
atoms remain in place, while the two ends are folded in opposite directions.
As a result, an axial substituent in one chair form of cyclohexane becomes an
equatorial substituent in the ring-flipped chair form, and vice versa.
3.2 ISOMERISM
41