4.7: Fischer Projections
Learning to draw Fischer projections of molecules and understanding their relevance plays a crucial role in the visual depiction of organic molecules. A Fischer projection is a two-dimensional projection on a planar surface to simplify the three-dimensional wedge–dash representation of molecules. This is especially helpful in the case of molecules with multiple chiral centers that can be difficult to draw. Here, all the bonds of interest are represented as horizontal or vertical lines. While the vertical lines represent an orientation away from the viewer, the horizontal lines indicate the groups directed towards the viewer. While molecules with a single chiral center can be transformed to their Fischer projections in a single step, converting the wedge–dash representation of a molecule with multiple chiral centers into its Fischer projection is a two-fold process.
First, the molecule is rotated to orient the carbon chain from top-to-bottom, with C-1 at the top. Next, the configuration at the lowest-numbered chiral center is visualized such that the substituents point towards the viewer and the carbon backbone is slanted away. Repeating this systematically for all chiral centers generates the Fischer projection of the entire molecule. A rotation of the molecule in the plane of the Fischer projection by 180° makes no difference, but a 180° rotation out of the plane of the projection generates the molecule’s enantiomer. One must note that the Fischer projections are just a simple 2D representation. They do not directly correlate to the actual 3D spatial structure of the molecule.
Figure 1: Different representation of a glucose molecule: (a) Fischer projection, (b) Wedge–dash, (c) Haworth projection, and (d) Chair conformation
While Fischer projections are commonly used to depict sugars in an open-chain form, the Haworth projections are typically used to depict their cyclic forms. For the Fischer projection of cyclic glucose molecule in Figure 1(a), the corresponding Haworth projection is presented in Figure 1(c). It should be noted that although Haworth projection is convenient to present stereochemistry, it fails to provide a realistic measure of the conformation. Therefore, to emphasize both conformation and stereochemistry in a molecule, the chair presentation is used (depicted in Figure 1(d)).