Overview

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)).

Procedure

The three-dimensional wedge–dash representation of molecules with multiple chiral centers can be very complicated and difficult to draw.

Such molecules can be more easily represented through their simple two-dimensional projections on a planar surface, known as Fischer projections.

In Fischer projections, all bonds of interest are represented as horizontal or vertical lines. While the vertical lines point away from the viewer, the horizontal lines are directed towards the viewer.

A molecule with only one chiral center, such as glyceraldehyde, can be easily rotated such that the carbon backbone points away from the viewer and the other substituents at the chiral center point towards the viewer. The projection of this orientation onto a plane creates the Fischer projection of glyceraldehyde.

In molecules with multiple chiral centers, such as ribose, there is no rotation possible such that all substituents at the chiral centers project towards the viewer while the carbon backbone points away from the viewer.

The Fischer projection of such a molecule is generated in two steps. First, the molecule is rotated such that the carbon chain is oriented top-to-bottom, conventionally with C-1 at the top.

Second, 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.

This orientation generates the Fischer projection for that chiral center. Repeating this process for each chiral center in the molecule creates the Fischer projection of the molecule.

However, the Fischer projection is only a simplistic two-dimensional representation and does not correlate directly to the actual three-dimensional structure of the molecule.

While rotation of the Fischer projection of a molecule in plane by one hundred and eighty degrees has no effect on the overall representation, an out-of-plane rotation by one hundred and eighty degrees creates the Fischer projection of the enantiomer of the original molecule.