ERM Ch. 3 Conformations Sorrell Su19.ppt

ERM Ch. 3 Conformations Sorrell Su19.ppt - Chapter 3...

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Chapter 3 Conformation of Alkanes and Cycloalkanes
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2 Conformations are different arrangements of atoms that are interconverted by rotation about a single bond. Conformations of Acyclic Alkanes Chapter 3
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3 Names are given to two different conformations. In the eclipsed conformation, the C-H bonds on one carbon are directly aligned with the C-H bonds on the adjacent carbon. In the staggered conformation, the C-H bonds on one carbon bisect the H-C-H bond angle on the adjacent carbon. Rotation of 60° to ethane interconverts each conformation Eclipsed and Staggered Conformations Chapter 3
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4 The angle that separates a bond on one atom from a bond on an adjacent atom is called a dihedral angle . For ethane in the staggered conformation, the dihedral angle for the C-H bonds is 60°; for eclipsed ethane, it is 0°. Conformations and Dihedral Angle Newman Projection Chapter 3
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Chapter 3 5 Sigma Bonds and Bond Rotation The staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possible The drawing to the right is called a Newman projection The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other Each eclipsed H-H costs 4.0 kJ/mol
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Chapter 3 6 Staggered and eclipsed conformations of ethane interconvert rapidly at room temperature The potential energy diagram of the conformations of ethane shows that the staggered conformation is more stable than eclipsed by 12 kJ mol -1 or 2.9 kcal mol -1 Each eclipsed C-H contributes 4 kJ/mol or 1 kcal/mol Torsional Strain: repulsion between pairs of bonding electrons that restricts rotation
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Chapter 3 7 Propane Torsional barrier is 14 kJ/mol (3.4 kcal/mol) Eclipsed conformation has three interactions Two ethane-type hydrogen-hydrogen interactions (4.0 kJ/mol (1 kcal/mol)) One additional hydrogen-methyl interaction (6.0 kJ/mol (1.4 kcal/mol))
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Chapter 3 8 Butane The lowest conformation is the anti conformation The geometric arrangement around a carbon-carbon single bond in which the two largest substituents are 180º apart as viewed in a Newman projection Conformation in which the two methyl groups of butane are as far apart as possible
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Chapter 3 9 Butane Rotation around C2-C3 bond results in the eclipsed conformation where there are two CH 3 —H interactions and one H—H interaction Gauche conformation The conformation of butane in which the two methyl groups lie 60º apart as viewed in a Newman projection Higher energy than the anti conformation even though it has no eclipsing interactions This conformation has 3.8 kJ/mol (0.9 kcal/mol) steric strain Steric strain The repulsive interaction in a molecule when two groups are closer together than their atomic radii allow
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Chapter 3 10
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  • Fall '15
  • Chellappah Chanmugathas
  • Conformational isomerism, Chemical bond, Cyclohexane conformation, ring strain

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