Part Eight: Isomers

  1. Isomers Overview

    Isomers = compounds that are not the same but share the same molecular formula

    Isomer TypeExample
    Structural Isomers = constitutional isomers

    Compounds that have a difference in the order inwhichthe components are bonded

    Conformational Isomers

    Compounds that differ in the rotation about a single bond


    Compounds that differ in the way they are oriented


    Stereoisomers that are NOT mirroimages

    A-1Geometric Isomers

    Diastereomers that differ specifically regarding the orientation of the substituents of a double bond or ring in a cis or trans orientation


    Stereoisomers that are mirror images

  2. Conformational Isomers Analysis
    1. Newman projections = a way to depict the different arrangements formed by rotation about the single bond of a molecule
      1. For example, in this conventional drawing of butane, one can appreciate that the structure can be rotated about its single bond

      2. Understanding Newman projections:

        The following images of the traditional structure drawing and corresponding Newman projection are color coordinated to demonstrate their relationship:

    2. What are the different types of conformational isomers? Here we define them by using butane as an example. We will go through the conformers by starting in the totally eclipsed position and rotate about the single bond from there.

      1. Totally Eclipsed - methyl groups are aligned with one another
        1. There is torsional strain between groups
          • Repulsion of electron clouds
        2. There is steric hindrance
          • The interference between two bulky groups
        3. These are the least stable of the conformations
        4. They possess the highest potential energy of the conformations
      2. Gauche- When the methyl groups are staggered to the left or right of one another at a 60° angle
        1. More stable
        2. Lower potential energy
      3. Eclipsed- Eclipsed position, but methyl groups are not eclipsed and are at a 120° angle to one another
        1. Less stable
        2. Higher potential energy
      4. Anti- When the methyl groups are staggered to be completely opposite of one another and are at a 180° angle
        1. Most stable
        2. Least potential energy
    3. Chair conformations
      1. Cyclohexane can be drawn in its conventional form, or in a variety of 3-D like model drawings. An important such conformation is known as a chair conformation.

      2. The chair conformation can flip between two different positions:

      3. Positions on the chair conformation
        1. Equatorial: Position nearly parallel to the overall axis of the structure.
        2. Axial: Position nearly perpendicular to the overall axis of the structure.

      4. The chair conformations demonstrate the importance of minimizing repulsion between groups to increase stability of a molecule
      5. 1,3 di-axial versus 1,3 di-equatorial
        1. 1,3 di-axial= very unfavorable due to steric hindrance between bulky groups
        2. 1,3 di-equatorial = much more favorable as steric hindrance is minimized and the structure is much more stable

Part Eight Exercise Set


  • Use what you learned in Part Eight to complete the following exercise.
    • Below is a potential energy diagram for conformational isomers of butane.
    • Each point on the x-axis corresponds to a different conformational isomer.
    • Choose from the following images to place at the appropriate point on the x- axis below:

    • When ready click on click on any window to reveal the correct conformer.