1) The document discusses conformational analysis in organic chemistry, focusing on ethane, butane, and cyclic molecules. 2) In ethane, the staggered conformation is lowest in energy due to orbital overlap between filled and empty orbitals. Butane has several distinct conformations with varying energies. 3) Cyclohexane exhibits angle strain as the bond angles are further from 109 degrees than in a puckered structure. Acyclic molecules can have high energy due to 1,3-strain from consecutive gauche interactions.

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CH264/2
Organic Chemistry II
Conformational Analysis
Dr Andrew Marsh C515
a.marsh@warwick.ac.uk
Dr David J Fox B510
d.j.fox@warwick.ac.uk

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Today’s Lecture
1. Stabilisation of conformation in simple alkanes
2. Strain in acyclic and cyclic molecules
3. Cyclohexane

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Ethane
Ethane is able to exist in many different CONFORMATIONS because it has free
rotation around its central C-C bond. There are however two extreme conformers,
the eclipsed (left) and staggered (right).
H H
H
H
H
H
H
HH
H
H
H
view view
H
HH
H
H
H Eclipsed Staggered
H
HH
H H
H
Revision: CGW pp. 363 – 364

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Dihedral Angle
Ha
HH
H
Hb
H
view
Staggered
H
HbH
H H
Ha
φ
If we chose a pair of hydrogen atoms in a Newman projection of ethane, then
the angle made between these two atoms, φ is called the DIHEDRAL ANGLE.

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Dihedral Angle φ
H°
Hb
HH
Ha
H
H
Hb
HH
H
H
Ha
Hb
HH
H
Ha
H
Hb
HH
Ha
H
H
H
HbH
H H
Ha
Hb
HH
H H
Ha
H
HHb
H H
Ha
Potential Energy - Dihedral Angle
0o 60o
120o 180o 240o 300o 360o
2.8 kcal mol-1
Calculate rate constant at
298 K for a 3 kcal/mol
barrier, using
k = Ae(-Ea/RT)

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Stabilisation of conformation
Filled orbital has a stabilising interaction with empty antibonding orbital if
antiperiplanar
H
HH
H
H
H
H
HH
H H
H
antiperiplanar
H
H
H
H
H
H
Newman projection
HH
H H
H H
filled orbital (HOMO) - empty (LUMO) overlap
eclipsed
C-HLUMO
C-HHOMO
C-HLUMO
C-HHOMO
CGW p.364

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Butane
CGW p.365 – 366
There are now several distinct conformations with different energies which are
revealed by Newman projections.

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D ih e d r a l A n g le φ
H °
P o t e n t ia l E n e r g y - D ih e d r a l A n g le
C H 3
HH
H 3 C
H
H
H
C H 3H
H 3 C
H
H
H
HH 3 C
H 3 C
H
H
C H 3
HH
H 3 C
H
H
H
C H 3H
H H
C H 3
C H 3
HH
H H
C H 3
H
HH 3 C
H H
C H 3
0 o 6 0 o 1 2 0 o 1 8 0 o 2 4 0 o 3 0 0 o 3 6 0 o
0 .9 k c a l/ m o l
6 k c a l/ m o l
3 k c a l/ m o l
0

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Klyne-Prelog Nomenclature
CH3
HH
H3C
H
H
H
CH3H
H3C
H
H
H
CH3H
H H
CH3
CH3
HH
H H
CH3
CH3 groups: synperiplanar – synclinal – anticlinal antiperiplanar
gauche anti
H
CH3H
H H
CH3
SYN
ANTI
H
H3C
CH3
H
CH3H
H H
CH3
–+
H
CH3H
H H
CH3
PERIPLANAR
PERIPLANAR
CLINALCLINAL

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Energetic Costs for Steric Interactions
Type of interaction Energy (kJ mol-1
)
H - H steric strain (gauche / synclinal) 0
Methyl - methyl steric strain (gauche / synclinal) 3.6
H - H torsional strain (eclipsed) 3.6
Methyl - H steric and torsional strain (eclipsed) 5.3
Methyl - methyl steric and torsional strain (eclipsed) 13 – 21
Note sometimes you will see values in kcalmol-1
: 1 kcalmol-1
= 4.184 kJmol-1

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1,3-Strain in acyclic molecules
Syn-pentane (g+ g- pentane) interactions. So-called due to presence of two
consecutive differently arranged gauche interactions
Certain conformations of linear chains are higher energy than others: linear
lowest energy. Two gauche interactions in certain directions are much higher.
H3C CH3 H3C
linear pentane
all anti-
CH3
HH
H H
CH3
H3C
H
H
H H
g +
H3C
H3C
H3C
H
H
H H
g+ g-
CH3
H
H
HH
H2
C
close H ... H contact
H
syn-pentane
H3C
H H H H
H H
envelope cyclopentane

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Cyclic Molecules - Angle Strain
Note that the bond angles in a planar ring are always further from 109° than if the
ring is puckered.
24.5° 9.5° 0.5° -5° "angle strain" (divided between two ring bonds)
9.17 6.58 1.24 0.02
38.4 27.5 5.19 0.09
strain per CH2 group (kcal mol-1)
strain per CH2 group (kJ mol-1)
"angle strain" is (109° - ring angle)/2

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You should be able to:
(i) Draw Newman projections for ethane and butane.
(ii) Predict the lowest energy conformer for simple alkanes.
(iii) Discuss the role of orbital stabilisation of conformation
(iv) Discuss the role of 1,3 strain in conformation of acyclics
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