Chapter04 - The Shapes of Molecules 4. Stereochemistry of...

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1 4. Stereochemistry of Alkanes and Cycloalkanes Based on McMurry’s Organic Chemistry , 6 th edition, Chapter 4 2 The Shapes of Molecules ± The 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 , emphasis on the first syllable) ± The systematic study of the shapes molecules and properties from these shapes is stereochemistry ± The field of stereochemistry is one of the central parts of organic chemistry and includes many important topics 3 4.1 Conformations of Ethane ± Conformers interconvert rapidly and a structure is an average of conformers ± Molecular models are three dimensional objects that enable us to visualize conformers ± Representing three dimensional conformers in two dimensions is done with standard types of drawings 4 Representing Conformations ± Sawhorse representations show molecules at an angle, showing a molecular model ± C-C bonds are at an angle to the edge of the page and all C-H bonds are shown ± Newman projections show how the C-C bond would project end-on onto the paper ± Bonds to front carbon are lines going to the center ± Bonds to rear carbon are lines going to the edge of the circle 5 Ethane’s Conformations ± There barrier to rotation between conformations is small (12 kJ/mol; 2.9 kcal/mol) The most stable conformation of ethane has all six C–H bonds away from each other ( staggered ) ± The least stable conformation has all six C–H bonds as close as possible ( eclipsed ) in a Newman projection – energy due to torsional strain 6 4.2 Conformations of Propane ± Propane (C 3 H 8 ) torsional barrier around the carbon– carbon bonds 14 kJ/mol ± Eclipsed conformer of propane has two ethane-type H–H interactions and an interaction between C–H and C–C bond
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2 7 4.3 Conformations of Butane ± anti conformation has two methyl groups 180° away from each other ± Rotation around the C2–C3 gives eclipsed conformation ± Staggered conformation with methyl groups 60° apart is
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This note was uploaded on 04/18/2011 for the course CHM 151 taught by Professor Dong during the Winter '08 term at University of Toronto- Toronto.

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Chapter04 - The Shapes of Molecules 4. Stereochemistry of...

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