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
view
H
H
H
HH
H
H
H
H
H
H
H
H
H
Staggered
Eclipsed
view
H
H
H
H
H
H
Revision: CGW pp. 363 – 364
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Dihedral Angle
If we chose a pair of hydrogen atoms in a Newman projection of ethane, then
the angle made between these two atoms,  is called the DIHEDRAL ANGLE.
Ha
H
Hb
H
H
f
view
H
Ha
H
Hb
H
H
Staggered
H
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Potential Energy - Dihedral Angle
Ha H b
H Hb
H
H
H
H
H Hb
Ha
H
H
H
H a Hb
H
H
H
Ha
H
H
H
H
2.8 kcal mol-1
H°
Ha
Ha
H
Hb
H
H
H
H
H
H
H
0o
60o
Ha
Hb
H
Hb
120o
180o
H
Calculate rate constant at
298 K for a 3 kcal/mol
barrier, using
k = Ae(-Ea/RT)
H
H
240o
300o
360o
Dihedral Angle f
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Stabilisation of conformation
filled orbital (HOMO) - empty (LUMO) overlap
H
H
H
H
H
H
C-HHOMO
H
H
H
H
H
H
C-HLUMO
antiperiplanar
Newman projection
C-HHOMO
H
H
H
H
HH
C-HLUMO
H
H
H
H
H
H
eclipsed
Filled orbital has a stabilising interaction with empty antibonding orbital if
antiperiplanar
CGW p.364
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Butane
There are now several distinct conformations with different energies which are
revealed by Newman projections.
CGW p.365 – 366
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Potential Energy - Dihedral Angle
H3C CH3
H3C CH3
6 kcal/mol
H
H
H
H
H
H
H3C H
H3C H
H°
H
H
H
CH3
H
H
H3C
H
CH3
CH3
3 kcal/mol
H
H3C
CH3
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
0.9 kcal/mol
0
0o
60o
120o
180o
240o
300o
360o
Dihedral Angle f
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Klyne-Prelog Nomenclature
PERIPLANAR
CH3
H
CH3
SYN
H
CH3
CLINAL
H
H
CH3
H
CH3
CLINAL
H
ANTI
H
–
+
CH3
H
H
H
H
H
CH3
PERIPLANAR
H3C
H
CH3
H3C CH3
H
H
H
H
CH3 groups: synperiplanar
H
CH3
H
H
H
– synclinal
gauche CH264
CH3
H3C H
H
H
H
CH3
– anticlinal
H
H
H
H
CH3
antiperiplanar
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anti
Energetic Costs for Steric Interactions
Energy (kJ mol-1)
Type of interaction
H - H steric strain (gauche / synclinal)
0
Methyl - methyl steric strain
3.6
(gauche / synclinal)
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
Certain conformations of linear chains are higher energy than others: linear
lowest energy. Two gauche interactions in certain directions are much higher.
H
H
H
H
syn-pentane
H3C
CH3
H3C
CH3
H3C
all anti-
close H ... H contact
H
H
H
CH3
envelope cyclopentane
H3C
linear pentane
H
H
H
H3C
H2
C
H3C
H
H3C
H
H
H
H
CH3
H
H
H
H
H
H
H
g+
H
g+ g-
Syn-pentane (g+ g- pentane) interactions. So-called due to presence of two
consecutive differently arranged gauche interactions
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Cyclic Molecules - Angle Strain
24.5°
9.5°
0.5°
-5°
"angle strain" (divided between two ring bonds)
9.17
6.58
1.24
0.02
strain per CH2 group (kcal mol-1)
38.4
27.5
5.19
0.09
strain per CH2 group (kJ mol-1)
"angle strain" is (109° - ring angle)/2
Note that the bond angles in a planar ring are always further from 109° than if the
ring is puckered.
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Outputs
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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