Relative Configuration

Core Concepts

In this tutorial, you will learn about, along with how to determine, the relative configuration of a molecule.

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What is Relative Configuration

Relative configuration is the specific arrangement of atoms in a molecule; this can be in relation to other atoms on the same molecule or another form of the same molecule. This configuration is independent of R and S notations, meaning that the absolute configuration does not automatically give you the relative.

The notations of D and L are used to tell us about the relative configuration of the molecule compared to the enantiomers of glyceraldehyde. The prefix D is assigned to compounds with the same relative configuration as (+)-glyceraldehyde, and the prefix L is assigned to compounds with the same as (-)-glyceraldehyde. (Note that D and L are different from d- and l-, you can find more information on d- and l- in the racemic mixtures article.) The D-L naming system is used for naming naturally occurring compounds, such as sugars and amino acids.

Determining Relative Configuration

Before starting, we need to briefly discuss Fischer projections. When creating a Fischer projection, we want our chiral center on the plane of paper. The bonds that are in the plane of paper will be depicted as vertical lines. Alternatively, the bonds that are going out of the plane of the paper will be depicted as horizontal lines.

Step 1:

In our example, we are going to use the amino acid alanine. Before determining relative configuration, we want to determine the absolute configuration of the molecule. In this example, alanine has an R configuration.

Determining the absolute configuration of alanine.

Step 2:

After determining the absolute configuration, we want to create the Fischer projection of our molecule. To start, we will place the carbonyl group at the top of the Fischer projection and the sidechain at the bottom. Next, we randomly place the amine group and the hydrogen on either side of the Fischer projection.

Creating the Fischer projection of alanine.

Step 3:

Now, we will assign priority to the atoms in the projection based on their atomic number. In our example, nitrogen is priority one because it has the highest atomic number of 7. The carbon that is part of the carbonyl group has the next priority. When assigning priority in a Fischer projection, a clockwise arrow, like in this example, means that the configuration is S. On the contrary, if the arrow is going counterclockwise, the configuration is R. This is the opposite of how we assign absolute configuration, so it is important to remember the difference!

Assigning configuration for the Fischer projection of alanine. — Fischer projection with an S configuration

Step 4:

If your Fischer projection is now the same as the previously determined absolute configuration, then we can determine the relative configuration. If it is the opposite of the previously determined absolute configuration, then we swap the hydrogen and amine group. This switches the absolute configuration so we can then determine the relative configuration.

In our example, the absolute configuration we got was R, and the configuration we got from the Fischer projection was S. That means we need to swap the amine group and the hydrogen. Now the absolute configuration and the configuration of the Fischer projection are both R.

Switching the amine group and the hydrogen of the Fischer projection.

With the configurations matching, we can now determine the relative configuration. To do this, we look at where the amine group is within the molecule. If the amine group is on the right side of the molecule, like in our example, it has a D configuration. Alternatively, if the amine group is on the left side of the molecule, it has an L configuration.

Due to our example being an amino acid, we focused on the amine group to determine relative configuration. When examining a sugar, we focus on the hydroxyl group that is on the chiral carbon furthest away from the top.