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Chemistry LibreTexts

4.2: Fischer Projections to communicate Chirality

  • Page ID
    182870
  • Learning Objective

    • draw, interpret, and convert between perspective formulae and Fischer projections for chiral compounds

    Perspective Formulas and Fischer Projections

    So far, we have communicated the stereochemical orentation of compounds using the wedges and dashes of perspective formulas. For example, the perspective formula for (R)-Lactic acid is shown below.

    A Fischer projection is a convention used to depict stereochemistry in two dimensions. The horizontal bonds are seen as wedges and the vertical bonds are seen as dashed lines as shown below in the example below for glyceraldehyde.

    fischer proj example.png

    Converting between Perspective and Fischer Formulas

    It can be useful to convert between perspective formulas and Fischer projections. Below is one approach using (R)-lactic acid as an example..

    Step 1: Hold the molecule so that

    a) the chiral center is on the plane of the paper,

    b) two bonds are coming out of the plane of the paper and are on a horizontal plane,

    c) the two remaining bonds are going into the plane of the paper and are on a vertical plane

    Step 2: Push the two bonds coming out of the plane of the paper onto the plane of the paper.

    Step 3: Pull the two bonds going into the plane of the paper onto the plane of the paper.

    Step 4: Omit the chiral atom symbol for convenience. This is the Fischer Projection of (R)-Lactic acid.

    The stereochemical formula for (R)-lactic acid can be drawn using either method. To build this skill, we begin by drawing the structures and converting them step wise. Models can also be helpful. Eventually, we will be able to mentally conversion between these two structures.

    R lactic acid fischer and perpective.png

    Exercise 1

    1. Convert each compound into the alternate sterochemical structure (Perspective resonance arrow.png Fischer).

    ch6 sect 2 exer cpds.png

    Answer

    ch6 sect 2 exerc soln.png

    See also D,L-convention.