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4.2: Cis-Trans Isomerism in Cycloalkanes

  • Page ID
    67078
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    Objectives

    After completing this section, you should be able to

    1. recognize that a formula of the type Cn H2n may represent a cycloalkane.
    2. draw structural formulas that distinguish between cis and trans disubstituted cycloalkanes.
    3. construct models of cis and trans disubstituted cycloalkanes using ball-and-stick molecular models.
    Key Terms

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

    • cis-trans isomers
    • cycloalkane
    • stereoisomer
    Study Notes

    Compounds in which the carbon atoms are joined in a ring, and in which no multiple bonds or other functional groups are present, are called cycloalkanes. A cycloalkane containing only one ring will correspond to the general formula CnH2n, but alkenes containing only one double bond also correspond to this formula. Cycloalkanes containing more than one ring are not considered in this section.

    Stereoisomers are also observed in certain disubstituted (and higher substituted) cyclic compounds. Unlike the relatively flat molecules of alkenes, substituted cycloalkanes must be viewed as three-dimensional configurations in order to appreciate the spatial orientations of the substituents. By agreement, chemists use heavy, wedge-shaped bonds to indicate a substituent located above the average plane of the ring, and a hatched line for bonds to atoms or groups located below the ring. As in the case of the 2-butene stereoisomers, disubstituted cycloalkane stereoisomers may be designated by nomenclature prefixes such as cis and trans. The stereoisomeric 1,2-dibromocyclopentanes below are an example.


    Structures of cis and trans-1,2-dibromocyclopentane.

    In general, if any two sp3 carbons in a ring have two different substituent groups (not counting other ring atoms) stereoisomerism is possible. This is similar to the substitution pattern that gives rise to stereoisomers in alkenes; indeed, one might view a double bond as a two-membered ring. Four other examples of this kind of stereoisomerism in cyclic compounds are shown below.

    Structures of cis-1,3-dimethylcyclobutane; trans-5-bromo-1,4,5-trimethyl-1,3-cycloheptadiene; trans-2,4-dichloro-1,1-dimethylcyclohexane; and cis-3,5-divinylcyclopentene

    If more than two ring carbons have different substituents (not counting other ring atoms) the stereochemical notation distinguishing the various isomers becomes more complex. However, we can always state the relationship of any two substituents using cis or trans. For example, in the trisubstitutued cyclohexane below, we can say that the methyl group is cis to the ethyl group, and trans to the chlorine. We can also say that the ethyl group is trans to the chlorine. We cannot, however, designate the entire molecule as a cis or trans isomer.

     

    Exercises

    Exercise \(\PageIndex{1}\)

    Draw the following molecules:

    1. trans-1,3-dimethylcyclohexane
    2. trans-1,2-dibromocyclopentane
    3. cis-1,3-dichlorocyclobutane
    Answer

    clipboard_eb51631551a0509db186ef3553799cb06.png

    Exercise \(\PageIndex{2}\)

    Cis/Trans nomenclature can be used to describe the relative positioning of substituents on molecules with more complex ring structures. The molecule below is tesosterone, the primary male sex hormone. Is the OH and the adjacent methyl group cis or trans to each other? What can you deduce about the relative positions of the indicated hydrogens?

     

    Answer

    Both the OH and the methyl group have wedge bonds. This implies that they are both on the same side of the testosterone ring making them cis. Two of the hydrogens have wedge bonds while one has a wedge. This means two of the hydrogens are on one side of the testosterone ring while one is on the other side.

    Exercise \(\PageIndex{3}\)

    Name the following compounds:

     

     

     

    Answer

    Cis-1-Bromo-3-Chlorocyclobutane

    Trans-1,4-Dimethylcyclooctane

    Trans-1-Bromo-3-ethylcyclopentane

    Contributors and Attributions


    4.2: Cis-Trans Isomerism in Cycloalkanes is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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