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Unformatted text preview: CHP 6 Problems: 6.1-19,21,23-25,27 6 Stereochemistry I Stereochemistry defines the three-dimensional structure of organic molecules. Sterio- isomers have the same bonds or connectivity but the bonds are in a different three dimensional orientation. 6.1 Cis-Trans Isomers Due to the restricted rotation around the C=C bond, 2-butene has two different stereo- isomeric forms (cis and trans). The cis has both methyl groups on the same side of the double bond while the trans has the methyl groups on opposite sides of the double bond. Cis -2-butene is slightly less stable than trans-2-butene due to steric interactions, between the two methyl groups. The non-bonded atoms are too close together and repel each other. Figure 6.3 6.2 Designating the Configuration of cis-trans Isomers. The three-dimensional arrangement of groups about a stereocenter in a molecule is termed its configuration. While cis-trans isomers can be used to designate configurations, they can be ambiguous at times. In these cases we use the designations E and Z . When the higher priority substituents are on the same side a Z designation is used, when they are on opposite sides an E designation is used. Priorities of groups are assigned by the Cahn-Ingold-Prelog sequence rules....
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