CH264 1 CH264/2 Organic Chemistry II Conformational Analysis Dr Andrew Marsh C515 Dr David J Fox B510

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Presentation transcript:

CH264 1 CH264/2 Organic Chemistry II Conformational Analysis Dr Andrew Marsh C515 Dr David J Fox B510

CH264 2 Today’s Lecture 1. Stabilisation of conformation in simple alkanes 2. Strain in acyclic and cyclic molecules 3.Cyclohexane

CH264 3 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

CH264 4 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.

CH264 5

6 Stabilisation of conformation Filled orbital has a stabilising interaction with empty antibonding orbital if antiperiplanar CGW p.364

CH264 7 Butane CGW p.365 – 366 There are now several distinct conformations with different energies which are revealed by Newman projections.

CH264 8

9 Klyne-Prelog Nomenclature

CH Energetic Costs for Steric Interactions Type of interactionEnergy (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 = kJmol -1

CH ,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.

CH Cyclic Molecules - Angle Strain Note that the bond angles in a planar ring are always further from 109° than if the ring is puckered.

CH 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 Outputs