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organic chemistry4.ppt

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organic chemistry4.ppt
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4. Stereochemistry of Alkanes and Cycloalkanes,Based on McMurry’s Organic Chemistry, 6th edition, Chapter 4,The Shapes of Molecules,The three-dimensional shapes of molecules result from many forces A molecule may assume different shapes, calledconformations, that are in equilibrium at roomtemperature (the conformational isomers are calledconformers, emphasis on the first syllable) The systematic study of the shapes molecules andproperties from these shapes is stereochemistry The field of stereochemistry is one of the centralparts of organic chemistry and includes manyimportant topics,4.1 Conformations of Ethane,Conformers interconvertrapidly and a structureis an average ofconformers Molecular models are three dimensionalobjects that enable usto visualize conformers Representing threedimensional conformersin two dimensions isdone with standardtypes of drawings,Representing Conformations,Sawhorse representationsshow molecules at an angle,showing a molecular model C-C bonds are at anangle to the edge of thepage and all C-H bondsare shown Newman projections showhow the C-C bond wouldproject end-on onto thepaper Bonds to front carbon arelines going to the center Bonds to rear carbon arelines going to the edge ofthe circle,Ethane’s Conformations,There barrier to rotation between conformations is small (12kJ/mol; 2.9 kcal/mol) The most stable conformation of ethanehas all six C–H bonds away from each other (staggered) The least stable conformation has all six C–H bonds as close aspossible (eclipsed),4.2 Conformations of Propane,Propane (C3H8)torsional barrieraround the carbon–carbon bonds 14kJ/mol Eclipsed conformerof propane has twoethane-type H–Hinteractions and aninteraction betweenC–H and C–C bond,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,4.4 Stability of Cycloalkanes: The Baeyer Str
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