This document discusses molecular conformations and isomerism. It begins by explaining that molecules possess kinetic energy due to motion, and this energy allows for free rotation about single bonds, resulting in different three-dimensional arrangements called conformations. The individual structures from free rotation are known as conformers. It then discusses various conformations like staggered, eclipsed, and skew, providing examples from ethane and butane. The document also discusses cycloalkanes and their conformations like chair, boat and half-chair. It explains concepts like axial and equatorial hydrogens in cyclohexane chair conformations and how substituents prefer equatorial positions to minimize 1,3-diaxial interactions.

1. © Dr. Atul R. Bendale

2. Molecules possess kinetic energy due to a state of continuous
motion.
The energy is transferred among molecules during collisions and is
sufficient to bring about rotation about single bond and for this
reason the rotation is termed free rotation.
Different three dimensional arrangements of atoms that result due
to free rotation about carbon–carbon single bond are known as
conformations.
The individual structures arising due to free rotation are known as
conformers or conformational isomers

3. In staggered conformations, the carbon–hydrogen (C–H) bonds on
each carbon are at a maximum distance and thus, have minimum
repulsion.
In eclipsed conformation, the carbon–hydrogen (C–H) bonds on
each carbon are at a minimum distance, that is, very close to each
other and thus, experience maximum repulsion.

4. 1. Sawhorse projection:
sawhorse projection, the two carbons attached through σ bond are
represented by points where four lines intersect.

5. In a staggered conformation, if we hold the front carbon and rotate the
rear carbon through an angle of 60° around carbon–carbon axis, it results
in an eclipsed conformation; a further rotation by 60° will result in a
staggered conformation.
2. Newman projection:
In Newman projection, a dot and a circle represent the two carbons,
attached through σ bonds. The represents the two carbons attached
through σ bond. The three bonds attached to front carbon are
represented as full lines

6. The study of the energy associated with different conformations is
known as conformational analysis. A plot between energy (along Y
axis) and angle of rotation (along X-axis) depicts that in the energy
diagram the lowest energy conformations are staggered
conformations. The maximum energy is associated with eclipsed
conformations.
The difference in energy between the most stable staggered and
least stable eclipsed conformation is referred to as torsional strain
of the molecule.

7. For ethane, the torsional strain is 3.0 kcal mol–1. The energy
required to overcome torsional strain is known as energy of
activation (Ea) and is supplied by molecular collision. The lower the
value of Ea, faster is the rotation about carbon–carbon single bond.

8. .
Conformations of Butane:
The Newman projections (I-VI) for different conformers of butane,
as obtained through 60° rotation around C2–C3 bond

9. Anti-conformation [conformation–I ]:
In an anti-conformation, the two CH3 groups are farthest from each
other and have minimum interaction.
Thus, the anti- form is the most stable conformation.
Eclipsed conformation [conformation–IV ]:
the methyl group on front carbon is exactly in front of methyl group
on the rear carbon.
The two bulkier methyl groups are very close to each other and this
steric crowding causes van der Waals repulsion.
The torsional strain and van der Waals repulsion together make the
totally eclipsed conformation least stable.
Skew conformations [conformations–II, III, V, and VI]:
Different conformations in butane which result from rotation about
carbon– carbon bond as one moves from totally staggered to
totally eclipsed conformation, are known as skew conformations

10. (i) Skew staggered conformation [Guache] (Conformations III and
V):
The staggered conformations in which two methyl groups are not
farthest apart but are at an angle of 60°, are known as Gauche
conformations. The gauche forms III and V are related as non-
superimposable mirror images and are termed as conformational
enantiomers.
(ii) Skew eclipsed conformations [Partially Eclipsed] (Conformations
II and VI):
In these conformations, methyl group and hydrogen on adjacent
carbon are present exactly in front of each other.
The eclipsed conformation experiences torsional strain as well as the
van der Waals repulsion between methyl group and hydrogen.

11. The skew eclipsed conformations II and VI are mirror images of each other
and are termed conformational enantiomers.
The skew eclipsed and skew staggered conformations do not exhibit
object–mirror image relationship and are termed conformational
diastereomers.

12. CYCLOALKANES:
CONFORMATIONS AND GEOMETRICAL ISOMERISM
Cycloalkanes have sp3 hybridized carbons and thus, they should
have a bond angle of 109.5°.
The simplest cycloalkane is cyclopropane which has a shape of
regular triangle with bond angles of 60°.
The sp3–sp3 overlap in cyclopropane is not as effective as in open
chain compounds.
The carbon–carbon bonds are bent and relatively weak which
makes the ring less stable. Cyclopropane is a planar molecule and
all the six hydrogens are present in an eclipsed state.
A Cyclopropane exhibits torsional strain as well as angle strain and
these two strains together cause ring strain in cyclopropane

13. In a planar cyclic structure, all the hydrogens are eclipsed which
leads to torsional strain.
The more the number of ‘eclipsed hydrogens’, higher is the
torsional strain.
The non-planar cyclopentane ring has a negligible angle strain and
is therefore more stable compared to cyclobutane and
cyclopropane.
Cis, trans isomerism in cycloalkanes:
cis-, trans-isomerism arises due to restricted rotation in double
bonds. Cyclic compounds can also have cis-, trans-isomerism
because the cyclic system prevents free rotation about single bond.

14. The substituted cycloalkanes posses chiral centre and exist as
enantiomers. Some of the isomers posses the plane of symmetry as
shown in the figure below and thus, do not exist as enantiomers
Conformations of Cyclohexane: The four main conformations of
cyclohexane:

15. 1. Chair conformation:
This is the most stable conformation of cyclohexane as it is free
from torsional strain.
All the twelve hydrogens are in staggered state as evident form
Newman projection.
The bond angle is nearly 109.5° and thus, it is free from angle
strain also.

16. 2. Boat conformation:
Twisting about carbon–carbon single bond of the chair form results
in the formation of boat conformation.
Boat conformer is free from angle strain.
However, in boat conformation the hydrogens are in eclipsed state,
which causes torsional strain in the molecule.

17. 3. Twist boat conformation
The boat conformation is flexible and a slight twist about the
bond reduces the torsional as well as flagpole interactions which
makes the twist boat conformation a little more stable than the boat
conformation.
Half chair conformation This is the least stable conformation of
cyclohexane because carbon atoms at one end of the ring are
planar.
Along with this the hydrogen at C1 and C4 are close to each other
and experience van der Waals repulsion known as flagpole
interaction.
The torsional strain and flagpole interaction make boat
conformation less stable compared to chair conformation

18. The stability order of different conformations of cyclohexane is:
Chair conformation >> twist boat conformation > boat
conformation > half chair conformation

19. The conformational anaylsis of cyclohexane is

20. Axial and equatorial hydrogens in chair conformation In the chair
conformation of cyclohexane, all the twelve hydrogens are not
equivalent. The chair conformation has two types of hydrogens,
axial (a) and equatorial (e).
Six hydrogens are present perpendicular to the plane of the ring
and are termed as axial hydrogens while the remaining six
hydrogens project out sideways, along the plane of the ring and are
termed as equatorial hydrogens

21. Each carbon has one axial and one equatorial hydrogen which point
in opposite directions.
The three axial hydrogens are perpendicular in upward direction
and three axial hydrogens are perpendicular in downward
direction.
The axial hydrogens point alternatively in upward and downward
directions in accordance to the vertices of the cyclohexane ring.
If the carbon (vertex) of the chair conformation is in upward
direction, the axial hydrogen will also be in upward direction,
however the equatorial hydrogen will be in downward direction,
however the equatorial hydrogen will be in downward direction.
A flip in chair conformation interconverts the axial and equatorial
hydrogens

23. If one hydrogen atom of cyclohexane is replaced by a larger atom
or group, the molecule becomes highly hindered.
As a result the repulsion between atoms increases.
Axial atoms/groups usually face more repulsive interaction in
comparison to equatorial atoms/groups.
The repulsive interaction experienced by three axial atoms is called
1,3-diaxial interaction. To minimize the 1,3-diaxial interaction and
resulting repulsive energy, the monosubstituted cyclohexane
acquires a chair conformation in which the substituents occupies
an equatorial position.
CONFORMATION OF MONO SUBSTITUTED CYCLOHEXANE

24. There are two possible chair conformations for methyl cyclohexane.
In one conformation the methyl group located at axial position (I),
whereas in other conformation the methyl group is located at
equatorial position (II).
When methyl group is at axial position, it has 1,3-diaxial
interaction with hydrogen atoms at C3 and C5 carbons due to which
the energy of such conformation is very high in comparison to the
conformer in which the methyl group is at equatorial position.
The conformer with methyl group at equatorial position does not
have any kind of 1,3-diaxial interaction hence is more stable.

25. DIFFERENCE BETWEEN CONFIGURATION AND CONFORMATION:
We have used the term conformer to explain isomers related to the rotation
about C-C single bond of ethane and butane derivatives, and the term
configuration to define some substituted methane and ethylene.
At first glance it seems straightforward to distinguish conformation and
configuration.
The stereoisomerism which is due to the rotation about a single bond is
referred to as conformation.
Conformers are easily interconvertible and it is difficult to isolate the isomer.
On the other hand, when two compounds are different in their
configuration, e.g., a pair of enantiomers of bromofluoromethane, or a pair
of geometrical isomers, maleic acid and fumaric acid, these are
distinguishable compounds, and their isolation is possible

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