a flat or planar structure, the bond angles will be 90
, so the angle strain (cf.
109.5
) will be much less than that of cyclopropane. However, cyclobutane in
its planar form will give rise to torsional strain, since all H atoms are eclipsed.
H
H
H
H
H
H
H
H
Cyclobutane as a planar molecule
All H atoms are eclipsed
Bond angle = 90 o
H
H
H
H
H
H
H
H
Most stable folded conformation of cyclobutane
H atoms are not eclipsed
Cyclobutane, in fact, is not a planar molecule. To reduce torsional strain,
this compound attains the above nonplanar folded conformation. Hydrogen
atoms are not eclipsed in this conformation and torsional strain is much less
than in the planar structure. However, in this form angles are less than 90
,
which means a slight increase in angle strain.
Conformational isomerism in cyclopentane Cyclopentane is a fivecarbon cyclic alkane. If we consider cyclopropane as a planar and regular
pentagon, the angles are 108
. Therefore, there is very little or almost no
angle strain (cf. 109.5
for sp
3 hybrids). However, in this form the torsional
strain is very large, because most of its hydrogen atoms are eclipsed. Thus,
to reduce torsional strain, cyclopentane twists to adopt a puckered or
envelope shaped, nonplanar conformation that strikes a balance between
increased angle strain and decreased torsional strain. In this conformation,
most of the hydrogen atoms are almost staggered.
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cyclopentane as a planar molecule
Bond angle = 108 o
Most stable puckered conformation of cyclopentane
Most H atoms are nearly staggered
Conformational isomerism in cyclohexane Cyclohexane (C 6 H 12 ) is a
six-carbon cyclic alkane that occurs extensively in nature. Many pharmaceutically important compounds possess cyclohexane rings, e.g. steroidal
molecules. If we consider cyclohexane as a planar and regular hexagon, the
angles are 120
(cf. 109.5
for sp
3 hybrids).
H
H H H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cyclohexane as a planar molecule
Bond angle 120 o
Most stable chair conformation
of cyclohexane
All neighbouring C-H bonds
are staggered
Boat conformation of cyclohexane
1
2
3
4
5
6
40
CH3 STEREOCHEMISTRY
, so the angle strain (cf.
109.5
) will be much less than that of cyclopropane. However, cyclobutane in
its planar form will give rise to torsional strain, since all H atoms are eclipsed.
H
H
H
H
H
H
H
H
Cyclobutane as a planar molecule
All H atoms are eclipsed
Bond angle = 90 o
H
H
H
H
H
H
H
H
Most stable folded conformation of cyclobutane
H atoms are not eclipsed
Cyclobutane, in fact, is not a planar molecule. To reduce torsional strain,
this compound attains the above nonplanar folded conformation. Hydrogen
atoms are not eclipsed in this conformation and torsional strain is much less
than in the planar structure. However, in this form angles are less than 90
,
which means a slight increase in angle strain.
Conformational isomerism in cyclopentane Cyclopentane is a fivecarbon cyclic alkane. If we consider cyclopropane as a planar and regular
pentagon, the angles are 108
. Therefore, there is very little or almost no
angle strain (cf. 109.5
for sp
3 hybrids). However, in this form the torsional
strain is very large, because most of its hydrogen atoms are eclipsed. Thus,
to reduce torsional strain, cyclopentane twists to adopt a puckered or
envelope shaped, nonplanar conformation that strikes a balance between
increased angle strain and decreased torsional strain. In this conformation,
most of the hydrogen atoms are almost staggered.
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cyclopentane as a planar molecule
Bond angle = 108 o
Most stable puckered conformation of cyclopentane
Most H atoms are nearly staggered
Conformational isomerism in cyclohexane Cyclohexane (C 6 H 12 ) is a
six-carbon cyclic alkane that occurs extensively in nature. Many pharmaceutically important compounds possess cyclohexane rings, e.g. steroidal
molecules. If we consider cyclohexane as a planar and regular hexagon, the
angles are 120
(cf. 109.5
for sp
3 hybrids).
H
H H H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Cyclohexane as a planar molecule
Bond angle 120 o
Most stable chair conformation
of cyclohexane
All neighbouring C-H bonds
are staggered
Boat conformation of cyclohexane
1
2
3
4
5
6
40
CH3 STEREOCHEMISTRY