Conformational Analysis
SIGMA BONDS AND BOND ROTATION
1. Two groups bonded by only a single bond can undergo rotation about that bond with respect
to each other
2. The temporary molecular shapes that result from
such rotation are called conformations.
3. Each possible structure is called a conformer
4. The analysis of the energy changes that a molecule undergoes as groups rotate about single
bonds known as Conformational analysis.
5. When we do confirmational analysis, we will find that certain kind of structural formulas are
convenient to use such as Newman projection formula and sawhorse formula. In conformational
analysis we will use substantial use of Newman projections.
Conformational Analysis
In a Newman projection the carbon–carbon bond is viewed along its axis and
the two carbon atoms are repesented by a circle. The bonds attached to the
front carbon are represented by lines going to the centre of the circle, and
bonds attached to the rear carbon are represented by lines going to the edge
of the circle. The advantages of Newman projections are that they are easy
to draw and they clearly show the relationships among substituents on the
different carbon atoms.
Conformational Analysis of Ethane
Potential energy changes
that accompany rotation
of groups about the
carbon-carbon bond of
ethane
Staggered conformation: allows the maximum Eclipsed conformation: maximum repulsive
separation of the electron pairs of the six C—H interaction between the electron pairs of the
bonds ⇒ has the lowest energy ⇒ most stable six C—H bonds (torsional strain) ⇒ has the
conformation. highest energy ⇒ least stable conformation.
http://www.kshitij-iitjee.com/Conformations-of-Alkanes/
Conformational Analysis of Ethane
Conformational Analysis of Butane
A C E G
All staggered conformations
have less energy than all
eclipsed conformations.
Staggered are free of
torsional strain thus more
stable
B D F
Conformational Analysis of Butane
What is steric strain:
A C E G Repulsive interaction between 2
groups (or atoms) that are forces
to be close to each other (close
enough to intrude to each
other’s space)
B D F
There are 2 types of eclipsed conformations in butane:
1) Eclipsed when the methyl groups are aligned (A and G)
2) Eclipsed when methyl groups aligned with hydrogen atoms (C and E)
All eclipsed conformations contains torsional strain.
The eclipsed conformations C and E have high energy due to steric strain arising from the eclipsed methyl groups
and hydrogen atoms.
The eclipsed conformation A and G have has the greatest energy due to the large steric strain arising from the
eclipsed methyl groups.
Conformational Analysis of Cycloalkane
The carbon atoms of alkanes are sp3 hybridized ⇒ the bond angle is
109.5°.
Ring Strain
Small cycloalkanes (3C or 4C) are very unstable.
There are 2 types of ring strain:
Angle strain is the increase in potential energy of a molecule
due to bond angles deviating from the ideal values (109.5ᵒ).
Tortional strain is the increase in potential energy of a
molecule due to repulsion between electrons in bonds that
do not share an atom.
Suggested Youtube link:
https://video.search.yahoo.com/search/video?fr=mcafee&p=ring+strain
#action=view&id=1&vid=b7226381c5313657ca56980a35009cfb
Bond angle: 120ᵒ
Planar conformer
All 12 C-H bonds
are eclipsed
(unstable)
Bond angle: 109.5ᵒ
(tetrahedral bond
angle)
Conformation of
Cyclohexane 12 C-H bonds are
Chair conformer
staggered/ gauche
no angle strain and
no torsional strain
(stable)
12 C-H bonds are
Boat conformer eclipsed (less
stable)
Preferred conformation in
which cyclohexane systems
exist in.
Looking at the chair
conformation, one can
identify a back-rest, a seat
and a leg-rest like that of a
chair.
1. The C—C bond angles are all 109.5° ⇒ free of angle strain.
2. Chair cyclohexane is free of torsional strain:
3. When viewed along any C—C bond, the atoms are seen to be
perfectly staggered.
4. The hydrogen atoms at opposite corners (C1 and C4) of the
cyclohexane ring are maximally separated.
• Boat conformations are also possible with cyclohexane
systems, but they are of higher energy than the chair
conformations.
Boat conformation of cyclohexane:
1) Boat conformation of cyclohexane is
free of angle strain.
2) Boat cyclohexane has torsional strain
and flagpole interaction
i) When viewed along the C—C bond on
either side, the atoms are found to
be eclipsed ⇒ considerable torsional
strain.
ii) The hydrogen atoms at opposite
corners (C1 and C4) of the cyclohexane
ring are close enough to cause van der
Waals repulsion ⇒ flagpole interaction.
Of all conformations of cyclohexane, the chair conformations are
of least energy and thus most stable. Chair conformations are
therefore the most realistic representations of cyclohexanes.
There are two types of bonds in cyclohexanes: Equatorial bonds are oriented
towards the rings equator, while axial bonds are on the rings axis.
To ring flip, identify where the substituents are. In one
conformer, push up at C-4 and down at C-1 to obtain the
4
C1 conformer. The other conformer is obtained by
pushing up at C-1 and down at C-1 to provide the 1C4
conformer. All equatorial bonds becomes axial and vise
versa when the ring flip.
• In methylcyclohexane, the methyl group can be in an
equatorial or axial position.
• We might, therefore, expect to find two conformers
of methylcyclohexane.
Stability of Cyclohexane Conformer
• Although cyclohexane rings rapidly flip between
conformations at room temperature, the two
conformers of a monosubstituted cyclohexane are
not of equal stability.
A B
•In the 1C4 conformer, there is van der Waals strain (steric) between
hydrogens of the axial CH3 and hydrogens at C-3 and C-5, while in
the 4C1 conformer, there is a smaller van der Waals strain between
hydrogens at C-1 and hydrogens at C-3 and C-5.
•This type of steric strain, because it arises from interaction between
an axial group on carbon 1 and an axial hydrogen on carbon atom 3
(or 5), is called 1,3-diaxial interaction (the interaction between
group/atom on carbon 1 with group/atom 3 carbon away)
•Generally there is less repulsion when any group is equatorial
rather than axial, thus conformer B is more stable than A
The investigation of molecular conformations and their relative
energies is called conformational analysis. As a generalization, for
other monosubstituted cycloalkanes: “a substituent is almost
always stable in an equatorial position than in an axial position”.
Conformational Structures of Disubstituted Cyclohexanes
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/sterism2.htm#:~:text=It%20is%20important%20to%20remember,always%20alternate%20in%20this%20fashion.&text=B
ecause%20of%20the%20alternating%20nature,trans%20is%20equatorial%2Faxial).
