Chapter04- Stereochemistry of Alkanes and Cycloalkanes

Chapter04- Stereochemistry of Alkanes and Cycloalkanes - 4...

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4. Stereochemistry of Alkanes and Cycloalkanes Based on McMurry’s Organic Chemistry , 6 th edition, Chapter 4 ©2003 Ronald Kluger Department of Chemistry University of Toronto
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McMurry Organic Chemistry 6th 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
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McMurry Organic Chemistry 6th edition Chapter 4 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
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McMurry Organic Chemistry 6th edition Chapter 4 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
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McMurry Organic Chemistry 6th edition Chapter 4 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
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McMurry Organic Chemistry 6th edition Chapter 4 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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McMurry Organic Chemistry 6th edition Chapter 4 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 gauche conformation
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