CH264/2 Organic Chemistry II Conformational Analysis

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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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Klyne-Prelog Nomenclature

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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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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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