Core Concepts
A Fischer projection is an illustration used to draw a three dimensional molecule in a two dimensional configuration. This is done by identifying chiral centers, determining stereochemistry, and drawing the corresponding flat molecule.
Introduction to Fischer Projection
Once chiral carbons have been identified, they must be transformed from three dimensional space to two dimensional space. Looking at the molecule with proper orientation, as thoroughly described in the subsequent steps, is a helpful technique to visualize the three dimensional molecule. Once this is completed, the molecule can be transformed to a flat model with assigned configuration. This is shown in the following four steps.
Fischer projections are the main model used to draw amino acids and carbohydrates. This is because they often contain more than one stereocenters. This is helpful for visualizing enantiomers.
Aside from Fischer projections, there are other models that depict stereochemistry. One example is a wedge/dash structure. Wedge/dash structures imply the wedge is going towards you and the dash is going away from you. Fischer projections imply horizontal lines are towards you and vertical lines are away from you. These structures look at the molecule from a different angle and are therefore drawn differently. A second example is newsman projections. Newman projections can be drawn eclipsed or staggered. However Fischer projections always depict the molecule eclipsed.
Step 1: Identify the Chiral Carbon
The first step to drawing a Fischer projection is finding all of a molecule’s chiral centers. Recall a chiral center is when an atom, typically a carbon, is attached to four different substituents. The image below shows examples of chiral and non-chiral centers.
If this was our example molecule, the carbon circled in yellow would be chiral, as it is attached to four different substituents.
Step 2: Visualizing the Molecule
The image below shows how to visualize the molecule to ensure proper stereochemistry.
As we can see, the eye line goes through the chiral carbon and the carbonyl carbon. This implies the OH and the H groups need to be drawn with three dimensional stereochemistry as shown below.
Step 3: Translate to 2D
Now that we can fully visualize stereochemistry, we must translate this into a two dimensional model. This is the general format used to do so.
Step 4: R/S Configuration
To assign R or S configuration, the four different substituents are labeled 1-4. 1 is the highest priority or the highest atomic number. 4 is the least priority or smallest atomic number.
For example, O has an atomic number of 8 and H has an atomic number of 1, so O is higher priority than H.
The group labeled 2 is higher priority than the group labeled 3 even though they are both carbons. This is because one carbon is attached to three hydrogens and one carbon is attached to two hydrogens and one carbon and a carbonyl group. Adding those atomic numbers you get for the methyl group is 6 + 1 + 1 + 1 = 9. The atomic numbers for the other group are 6 + 1 + 1 + 6 + 1 + 8 = 23. Clearly, the carbon attached to a carbonyl takes higher priority.
Drawing an arrow as shown below indicates this molecule is clockwise, or R configuration.
Since the oxygen is on the right and it is higher priority than the hydrogen, it can also be called D.
If the molecule was a counterclockwise circle is would be S configuration. If the hydrogen was on the right and the oxygen was on the left, it would be called L.
Enantiomers and Diastereomers
Drawing molecules with different orientation creates enantiomers and diastereomers. Enantiomers are nonsuperimposable mirror images, such as the molecules labeled I and II. Diastereomers are nonsuperimposable and non-mirror images, such as the molecules labeled I and III or I and IV. Although all four molecules have the same atoms, their spatial arrangement makes this different.
Conclusion
Fischer projections are a useful tool to visualize molecules on a flat surface to accurately depict their location in space and compare enantiomers and diastereomers.