Chapter 3 - Ch Chapter 3 Alkanes and Cycloalkanes...

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Chapter 3 Alkanes and Cycloalkanes: Conformations and cis - trans Stereoisomers Conformations are different spatial arrangements of a molecule that are generated by rotation about single bonds. Chem 211 B. R. Kaafarani 1
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3.1. Conformational Analysis of Ethane Eclipsed conformation Chem 211 B. R. Kaafarani 2
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Ethane Staggered conformation Chem 211 B. R. Kaafarani 3
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Projection Formulas of the Staggered Conformation of Ethane H H H H H H H H H H H H Newman Sawhorse Chem 211 B. R. Kaafarani 4
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Anti Relationships H H H H H H H H 180 ° H H H H Two bonds are anti when the angle between them Chem 211 B. R. Kaafarani 5 is 180 ° .
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Gauche Relationships H H 60 ° H H H H H H H H H H Two bonds are gauche when the angle between Chem 211 B. R. Kaafarani 6 them is 60 ° .
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n important point: An important point: The terms anti and gauche apply only to bonds (or groups) on adjacent carbons, and only to staggered conformations. Chem 211 B. R. Kaafarani 7
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12 kJ/mol 0 ° 60 ° 120 ° 180 ° 240 ° 300 ° 360 ° Chem 211 B. R. Kaafarani 8
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Torsional strain he clipsed onformation f thane 2 J/mol The eclipsed conformation of ethane is 12 kJ/mol less stable than the staggered. The eclipsed conformation is destabilized by torsional strain. Torsional strain is the destabilization that results from eclipsed bonds. Chem 211 B. R. Kaafarani 9
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2 3.2 Conformational Analysis of Butane Chem 211 B. R. Kaafarani 10
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4 kJ/mol 3 kJ/mol 14 kJ/mol 0 ° 60 ° 120 ° 180 ° 240 ° 300 ° 360 ° Chem 211 B. R. Kaafarani 11
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van der Waals strain The gauche conformation of butane is 3 kJ/mol less stable than the anti . The gauche conformation is destabilized by van der Waalsstrain (also called stericstrain). van der Waals strain is the destabilization that Chem 211 B. R. Kaafarani 12 results from atoms being too close together.
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van der Waals strain The conformation of butane in which the two methyl groups are eclipsed with each other is the least stable of all the conformations. It is destabilized by both torsional strain Chem 211 B. R. Kaafarani 13 (eclipsed bonds) and van der Waals strain.
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3.3. Conformations of Higher Alkanes Unbranched alkanes Hexane The most stable conformation of unbranched Chem 211 B. R. Kaafarani 14 alkanes has anti relationships between carbons.
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3.4. The Shapes of Cycloalkanes: lanar or Nonplanar? dolf von Baeyer (19 th entury) Planar or Nonplanar? Assumed cycloalkanes are planar polygons. Adolf von Baeyer (19 century) Distortion of bond angles from 109.5 ° gives angle strain to cycloalkanes with rings either smaller or larger than cyclopentane. Baeyer deserves credit for advancing the idea of angle strain as a destabilizing factor. But Baeyer was incorrect in his belief that Chem 211 B. R. Kaafarani 15 cycloalkanes were planar.
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Types of Strain Torsional strain: train that results from eclipsed bonds strain that results from eclipsed bonds.
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This note was uploaded on 01/06/2012 for the course CHEM 211 taught by Professor Bilalkaafarani during the Spring '11 term at American University of Beirut.

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Chapter 3 - Ch Chapter 3 Alkanes and Cycloalkanes...

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