Conformations of Cycloalkanes

Cycloalkanes are not always planar structures. Being planar requires large variations in
bond angles from the ideal value of 109.5º for a tetrahedral carbon.
 Cyclopropane

Cyclopropane
C l has
h a high
hi h degree
d off torsional
i l strain
i due
d to large
l number
b off eclipsing
li i
interactions.

109.5°

49.5°

60°

The large angular strain in cyclopropane is compensated partially through bent bonds
 Cyclobutane
H H
H H H
H H H

H H H H
H H
H H
C l b
Cyclobutane has
h eight
i h H---H
H H eclipsing
li i interactions
i i in
i a planar
l structure

109.5° H
H H
H
88°
19.5° H H
H
H
90°
Torsional strain in cyclobutane is reduced by adopting a slightly folded structure, which
results in additional angular strain
 Cyclopentane
A planar
l structure
t t f cyclopentane
for l t wouldld have
h b d angles
bond l off 108°,
108° which
hi h is
i very
close to the ideal values for a tetrahedral carbon atom. However, such a structure could
lead to considerable torsional strain resulting from ten H---H eclipsing interactions.

Torsional strain is reduced by moving one or two carbon away from the plane. This
results in an increase in angular strain. Carbon atoms move in and out of the plane
rapidly, resulting in an illusion of rotation of the molecule. This phenomenon is termed
as pseudo-rotation.
 Cyclohexane
Cyclohexane
C l h avoids
id torsional
t i l interactions
i t ti b adopting
by d ti non-planar
l conformations,
f ti
which also reduces the bond angles to that of a perfect tetrahedron (~109.5°).

The chair conformation of cyclohexane is the most stable. It has no torsional strain as
all the C-H bonds are staggered to each other.
other The bond angle is very close to the ideal
value.

The chair conformation can be viewed as having two carbon atoms on the plane of the
paper. Two in another plane in front of the paper and the remaining two in a third plane,
behind the plane of the paper
 Cyclohexane
Cyclohexane
C l h f
forms a number
b off different
diff t conformers.
f H
However, structures
t t other
th than
th
the chair conformation suffers from torsional strain, angular strain or both.

Boat Conformation: It has no angular strain. However, in addition to the torsional

strain resulting from 4 H---H interactions, it also has a flagpole interaction between the
hydrogen atoms on 1- and 4-carbon atoms.

Twist Conformation: It is more stable than the boat conformation, but less stable than
the
h chair
h i conformation.
f i The
Th flagpole
fl l interactions
i i andd torsional
i l strain
i in
i the
h boat
b
conformation are reduced in the twist conformer.
 Conformational Analysis of Cyclohexane

The cyclohexane continuously flips from one chair conformation to the other. other
Approximately 1 million such interconversions occur every second. More than 99% of
the molecules are estimated to be in a chair conformation at any given time.
Drawing the Chair Conformation of Cyclohexane

Opposite bonds are drawn parallel to each other.

 Axial and Equatorial Bonds
Axial Bonds: They are parallel to each other and to the principal axis, but
perpendicular to the average plane of the ring. There are three bonds facing up and
three
ee facing
c g dow
down..
Equatorial Bonds: These are three sets of two parallel bonds, each of which are
parallel to two of the bonds in the ring. Equatorial bonds alternate from slightly up to
slightly
g y down orientation on movingg from one carbon to the next

Axial (red) and equatorial (green)

bonds as viewed from the side (L) and
top (R).
 Axial and Equatorial Bonds
Each equatorial bond is parallel to two carbon-carbon bonds in the ring and another
equatorial bond.

Axial and equatorial bonds undergo interconversion, when a chair conformation flips to
the other. However, their relative orientations in space do not change.
4 2 3 1
3 2
E t i l
Equatorial
1 4

A i l
Axial
 Mono-substituted Cyclohexane

Any substitution bigger than hydrogen in an axial position leads to unfavorable

interactions with axial hydrogen atoms on the 3-carbon atom. Such an interaction is
generally termed as 1,3-diaxial interaction.
In the case of a methyl substituent, these interactions are exactly same as a gauche
butane interaction and amounts to 1.8 kcal/mol (two such interactions). In an
equatorial position, the methyl group is anti to C3 in the ring.
 Steric Strain in Mono-substituted Cyclohexanes

The relative population of the two chair conformers can be calculated by the equation,
ΔG = − RT ln Keq
For methylcyclohexane,
methylcyclohexane the concentration of the equatorial form is almost 95%,
95% while
for tert-butylcyclohexane the equatorial conformer is present in 99.99% at room
temperature.
 Di-substituted Cyclohexanes
Relative
R l ti orientation
i t ti off two
t substituents
b tit t in i a cycloalkane
l lk are represented
t d by
b cis
i andd
trans notations.
trans-1,2-dimethylcyclohexane

The di-axial conformer is unstable by 4 × 0.9 = 3.6 kcal/mol

The two equatorial methyl groups in the di-equatorial conformer are gauche to each
other and has a destabilizing interaction amounting to 0.9 kcal/mol
cis-1,2-dimethylcyclohexane

Both the conformers are destabilized by 1.8 + 0.9 = 2.7 kcal/mol

 Di-substituted Cyclohexanes
cis-1,3-dimethylcyclohexane

The di-axial conformer is veryy unstable due to two CH3 beingg in axial ppositions at the
same side.
The di-equatorial conformer has no gauche butane interactions and is stable.

trans-1,3-dimethylcyclohexane

Both the conformers are destabilized by 1.8 kcal/mol

 Di-substituted Cyclohexanes
cis-1,4-dimethylcyclohexane

Both the conformers are destabilized by 1.8

1 8 kcal/mol

trans-1,4-dimethylcyclohexane

The di-axial
Th di i l conformer
f i unstable
is bl by
b 4 × 0.9
0 9 = 3.6
3 6 kcal/mol
k l/ l
The di-equatorial conformer has no gauche butane interactions and is stable.