Chapter3 - Organic Chemistry CHE 275 Chapter 3...

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Organic Chemistry CHE 275 Chapter 3 Conformations of Alkanes and Cycloalkanes
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• Three-dimensional shapes of molecules result from many forces • A molecule may assume different shapes, called conformations , that are in equilibrium at room temperature • The conformational isomers are called conformers • The systematic study of the shape of molecules and the properties of each shape is called stereochemistry • The field of stereochemistry is one of the central parts of organic chemistry and includes many subtopics The Shapes of Molecules
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• Conformers interconvert rapidly • Molecular models are 3D objects that help us visualize the conformers • Representing 3D conformers in 2D is done with standard types of drawing Conformations of Ethane
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Sawhorse representations show molecules at an angle like in a molecular model • C-C bonds are at an angle to the edge of the page and all C-H bonds are shown Sawhorse Representations
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Newman projections show how the C-C bond would project end-on into the paper • Bonds to the front carbon are lines going to the center • Bonds to the rear carbon are lines going to the edge of the circle Newman Projections
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• The barrier to rotation between conformations in ethane is small (12 kJ/mol) • The most stable conformation of ethane has all 6 C-H bonds pointing away from each other ( staggered ) • The least stable conformation has all 6 C-H bonds as close as possible ( eclipsed ) - this is due to torsional strain Ethane's Conformations
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• The two bonds are anti when the angle between them is 180° Anti Relationships H H H H H H H H H H H H 180°
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• The two bonds are gauche when the angle between them is 60° The terms anti and gauche apply only to bonds or groups on adjacent carbons, and only to staggered conformations Gauche Relationships H H H H H H H H H H H H 60°
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Ethane's Conformations 60° 120° 180° 240° 300° 360° 12 kJ/mol
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• 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 Torsional Strain
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• Propane (C 3 H 8 ) has a higher barrier to rotation around the C-C bond than ethane (14 kJ/mol) • The eclipsed conformer of propane has two ethane-type H-H interactions and an interaction between the C-H and C-C bond Conformations of Propane
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• Butane has three conformations • The anti conformation has the two methyl groups oriented 180° away from each other • The eclipsed conformation has the two methyl groups in the same plane • The staggered conformation with the Me groups 60° apart is called the gauche conformation Conformations of Butane
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• The anti conformation of butane is the most stable • Both methyl groups bisect the hydrogens on the adjacent carbons • No torsional strain because there are no overlapping bonds Anti Butane
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• Gauche butane is 3 kJ/mol less stable than the anti conformation There is no torsional strain because no bonds overlap • The anti conformation is destabilized by steric strain Gauche Butane
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