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5.1: Prelude to Stereoisomerism

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    By now you should be familiar with position isomers wherein compounds of the same molecular formula differ because substituents, chain branches, and so on, are not at the same positions in the molecules. 1-Chloropropane and 2-chloropropane are straightforward examples of position isomers. A much more subtle form of isomerism is present when two different compounds have the same molecular formulas, the same substituent and chain-branching positions, and, indeed, even have the same names by all of the nomenclature rules we have given you so far. Such isomers are different because their molecules have different arrangements of the atoms in space. These are stereoisomers and this type of isomerism, called stereoisomerism, is of enormous importance to all areas of organic chemistry and biochemistry.

    To understand stereoisomerism of carbon compounds, we must understand the ways in which the bonds to carbon atoms are arranged in space. As shown in Section 2-2A, this depends on whether the carbon atoms form single, double, or triple bonds to another atom. Thus, four single bonds to a carbon form a tetrahedral arrangement; two single bonds and one double bond to a carbon give a planar array with bond angles near \(120^\text{o}\), while one single bond and one triple bond (or two double bonds) to a carbon are arranged linearly:

    From left to right: Tetrahedral; carbon with four bonds, two straight, one dashed and one wedged. Planar trigonal; carbon with two single bonds and one double bond. Linear; carbon with one triple bond and one single bond. Linear; carbon with two double bonds.

    Finally, if you have not studied the material already, you may wish to return to the last part of Chapter 3 and become acquainted with the nomenclature of cycloalkanes, alkenes, cycloalkenes, and alkynes (Sections 3-2 to 3-4).

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."


    This page titled 5.1: Prelude to Stereoisomerism is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.