Ch4slides - Chapter 4: Stereochemistry of Alkanes and...

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1 Chapter 4: Stereochemistry of Alkanes and Cycloalkanes Stereochemistry: three-dimensional aspects of molecules Conformation: different geometric arrangements of atoms that result from rotations about single ( σ ) bonds Conformer: a specific conformation of a molecule Conformations of Ethane CH 3 -CH 3 Staggered Eclipsed H H H H H H Front carbon Back carbon Sawhorse Newman projection C C H H H H H H
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2 Energy vs. Dihedral Angle for Ethane http://www2.chem.ucalgary.ca/Flash/ethane.html Torsional Strain: strain (increase in energy) due to eclipsing groups eclipsed staggered Conformations of Propane
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3 Staggered conformations Energy vs. Dihedral Angle for Propane Dihedral (torsional) angle 14 KJ/mol Conformations of Butane Staggered anti gauche Eclipsed CH 3 - H CH 3 - CH 3
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4 Steric Strain: repulsive interaction that occurs when two groups are closer than their atomic radii allow Energy vs Dihedral Angle for Butane http://www2.chem.ucalgary.ca/Flash/butane.html
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5 Strain Energy of Alkane Conformations H - H eclipsed 4.0 KJ/mol torsional strain H - CH 3 eclipsed 6.0 KJ/mol mostly torsional strain CH 3 - CH 3 eclipsed 11 KJ/mol torsional and steric strain CH 3 - CH 3 gauche 3.8 KJ/mol steric strain Torsional Strain: strain (increase in energy) due to eclipsing groups Steric Strain: repulsive interaction that occurs when two groups are closer than their atomic radii allow Heats of Combustion of Cycloalkane: the more strain a compound is, the more heat it released upon combustion C n H 2n + 3n/2 O 2 n CO 2 + n H 2 O + heat cycloalkane (can be measured) Total Strain Energy = Sample Δ H comb per -CH 2 - _ Reference Δ H comb per -CH 2 - • n Baeyer Strain Theory Angle Strain: strain due to deforming a bond angle from its ideal value
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This note was uploaded on 03/11/2012 for the course CHEM 220A taught by Professor Sulikowski during the Fall '08 term at Vanderbilt.

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Ch4slides - Chapter 4: Stereochemistry of Alkanes and...

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