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7.6: Polycyclic Alkanes

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  • Objective

    After completing this section, you should be able to draw the structures and construct molecular models of simple polycyclic molecules.

    Key Terms

    Make certain that you can define, and use in context, the key terms below.

    • bridgehead carbon atom
    • polycyclic molecule
    Study Notes

    A bridgehead carbon atom is a carbon atom which is shared by at least two rings. The hydrogen atom which is attached to a bridgehead carbon may be referred to as a bridgehead hydrogen.

    Note that bicyclo[2.2.1]heptane is the systematic name of norborane. You need not be concerned over the IUPAC name of norbornane. The nomenclature of compounds of this type is beyond the scope of this course.

    Nomenclature of Bicyclic Ring Systems

    There are many hydrocarbons and hydrocarbon derivatives with two rings having common carbon atoms. There are three main ways that the two rings can be connected. The first is called a fused bicyclic ring structure where the two rings share a covalent bond and a have two bridgehead carbons (marked in red on the structures below). A bridgehead is defined as a carbon that is part of two or more rings. Hydrogens attached to bridge head carbons are often referred to as bridge head hydrogens. The two rings can also be connected by a bridge containing one or more carbons to form a bridged bicyclic molecule. Lastly, the two rings can be joined with a singe bridge head carbon to form spiro bicyclic molecules.

    Fused, bridge, and spiro systems in bicyclic isomers.
    Bicyclic Isomers of C10H18

    Naming Fused and Bridged Compounds

    Fused and bridged bicyclic compounds are follow similar naming conventions:

    1. Count the total number of carbons in both rings. This is the parent name. (eg. ten carbons in the system would be decane)
    2. Count the number of carbons between the bridgeheads, then place the numbers in square brackets in descending order separated by periods. Fused and bridged bycyclic compounds should have three numbers such as [2.2.0]. For fused compounds one of the numbers should be zero.
    3. Place the word bicyclo at the beginning of the name.

    Bond line drawings of bicyclo[4.4.0] decane and bicyclo[4.3.1]decane with bridgehead carbons labeled.

    Examples with carbons and hydrogens explicitly shown:

    Lewis structure of bicyclo[2.2.1]heptane, bicyclo[3.3.1]heptane, and bicyclo[1.1.0]butane.

    Naming Spiro Compounds

    Spiro bicyclics are named using the same basic rules. Because there is only one bridgehead carbon only two numbers will be required in the brackets. Also, the word spiro is placed at the beginning.

    Bond line drawings of spiro[5.4]decane with atoms and bridgehead carbons labeled


    Bond line drawings of spiro[4.4]nonane and spiro[3.2]hexane

    Conformations in Bicyclic Ring SystemsEdit section

    As expected, the connection of two rings has defined effects on the possible conformations. However, the ideas previously discussed in this chapter can be used for conformational analysis. Fused rings have the possibility of two isomers where the bridgehead hydrogens are either cis or trans along the shared bond. These two isomers have significant differences in flexibility and stability as seen in bicyclo[4,4,0]decane more commonly known as decalin. If the positioning of the bridgehead hydrogens are shown in a fused ring the prefix cis or trans should be included in the name.

    The trans-isomer is the easiest to describe because the fusion of the two rings creates a rigid, roughly planar, structure made up of two chair conformations. Unlike cyclohexane, the two rings cannot flip from one chair form to another. Accordingly, the orientation of the any substituents is fixed in either an axial or equatorial position in trans-decalin. This means that the C-C bonds coming away from the fused edge are held in equatorial positions relative to each ring thus preventing the possibility of any 1,3-diaxial interactions occurring between ring atoms.

    Wedge-dash structure and chair conformation of trans-decalin.

    Interactive Element

    The 3D Structure of Trans-Decalin

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