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Shapes of Molecules with Double (and Triple) Bonds

  • Page ID
    52307
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    We can apply the same thinking about the arrangement of bonds around the carbon atoms in CH2=CH2 in much the same manner as we did for CH3–CH3. In ethene each carbon atom is surrounded by three centers of electron density, two Hs and one C. Note that the double bond counts as a single center of electron density. There are a number of important points to keep in mind when considering the effects of double bonds on a molecule and its properties. First, a C=C double bond is typically less stable (that is more reactive) than two separate single bonds. When we come to thinking about reactions we will find that replacing a double bond by two single bonds typically produces a more stable system. Second, although there is more or less free rotation around the axis of a single bond at room temperature, rotation is blocked by the presence of a double bond. For a rotation to occur, the π bond (in which there is electron density above and below the axis between the two carbon atoms) must be broken and then reformed. The presence of a double bond has distinct effects on molecular shape. The minimum energy arrangement for three centers is a two-dimensional arrangement in which the groups are oriented at about 120º to one another; an arrangement known as trigonal planar geometry.

    There is one more common type of bond that carbon can form, which is a triple bond. For example each carbon in C2H2 (ethyne) is surrounded by only two centers of electron density: a single sp hybrid orbital bonds between a carbon atom and a hydrogen atom and a triple bond, which can be thought of as a σ bond and two π bonds between the carbons, shown in the figure. The lowest energy arrangement around each carbon is a line in which the angle between the bonds is 180º. As before, a triple bond is less stable than three single bonds, and reactions can be expected!

    We see that under most conditions, a carbon atom can participate in a maximum of four bonds; either four single bonds, two single bonds and a double bond, or one single bond and a triple bond..

    Questions to Answer

    1. Given a particular hydrocarbon, what factors would influence your prediction of its melting and boiling points? Can you generate some tentative rules?

    2. How does the presence of a double bond influence the structure of a hydrocarbon?

    3. How is the presence a triple bond different from that of a double bond?

    4. Why do you think there is no tetrabonded from of carbon (that is C four bonds C).

    Questions to Ponder


    1. What limits the size and shape of a hydrocarbon?


    Shapes of Molecules with Double (and Triple) Bonds is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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