1. CH264 1 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

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

3. 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

4. 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.

5. CH264 5

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

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

8. CH264 8

9. 9 Klyne-Prelog Nomenclature

10. CH264 10 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 = 4.184 kJmol -1

11. CH264 11 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.

12. CH264 12 Cyclic Molecules - Angle Strain Note that the bond angles in a planar ring are always further from 109° than if the ring is puckered.

13. CH264 13 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