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Chemistry LibreTexts

2.6: Bond Rotation

  • Page ID
    182837
  • Learning Objective

    • distinguish between bonds that can rotate and those that cannot

    Sigma Bonds can Rotate

    We learned in section 2.1 that single bonds in organic molecules are free to rotate, due to the 'end-to-end' (sigma) nature of their orbital overlap. Consider the carbon-oxygen bond in ethanol, for example: with a 180o rotation about this bond, the shape of the molecule would look quite different:

    For ethane, rotation about the carbon-carbon sigma bond results in many different possible three-dimensional arrangements of the atoms.

    Pi Bonds are rigid

    Pi bonds are created from overlapping p orbitals. The lobes of the p orbitals prevent the atoms sharing pi bonds from rotating as shown in the diagram below.

    Double and Triple Bonds cannot Rotate

    The pi bonds in double and triple bonds prevent these bonds from rotating. This rigidity has an effect on the physical structure of compounds and can influence chemical reactivity. For now, we want to build the habit of looking at static drawings and diagrams of organic compounds and visualizing their dynamic nature.

    For ethene, there is no rotation about the carbon-carbon double bond because of the pi bond.

    ch 2 sect 6 example part 1.png

    Exercise

    1. Label the selected bonds in the compound below as "Rotates" or "Rigid."

    ch 2 sect 6 exercise.png

    Answer

    1. Arrows for (a) and (c) are pointing to single bonds that can rotate. Arrow (b) is pointing to a double bond that is rigid because of the pi bond.

    ch 2 sect 6 exercise solution.png