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CH264/2 Organic Chemistry II Conformational Analysis

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

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CH264/2 Organic Chemistry II Conformational Analysis

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  1. CH264/2 Organic Chemistry IIConformational Analysis Dr Andrew Marsh C515 a.marsh@warwick.ac.uk Dr David J Fox B510 d.j.fox@warwick.ac.uk 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. CH264

  6. Stabilisation of conformation Filled orbital has a stabilising interaction with empty antibonding orbital if antiperiplanar CGW p.364 CH264

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

  8. CH264

  9. Klyne-Prelog Nomenclature CH264

  10. 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 CH264

  11. 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. Syn-pentane (g+ g- pentane) interactions. So-called due to presence of two consecutive differently arranged gauche interactions 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. CH264

  13. Outputs You should be able to: Draw Newman projections for ethane and butane. Predict the lowest energy conformer for simple alkanes. Discuss the role of orbital stabilisation of conformation Discuss the role of 1,3 strain in conformation of acyclics CH264

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