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CH264/2 Organic Chemistry II
Conformational Analysis
Dr Andrew Marsh [email protected]
Dr David J Fox [email protected]
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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).
Revision: CGW pp. 363 – 364
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Dihedral AngleIf 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.
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Stabilisation of conformation
Filled orbital has a stabilising interaction with empty antibonding orbital if antiperiplanar
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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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.
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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.
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