How to Determine R and S Configuration (Stereochemistry)

What is R or S Configurations?

R and S (rectus and sinister respectively in Latin) are terms that describe the stereochemistry configuration of organic molecules. Stereochemistry is important because it changes the shape of a molecule, and even the reactions that it participates in. In order to effectively communicate a version (or isomer) of molecule, chemists invented a system for specifying these differences. In this article, we will show you how to classify chiral molecules as R or S.

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Looking For Chiral Centers

In order for a molecule to be chiral, it needs to have a chiral center. A chiral center in an organic molecule is a carbon atom with four different substituents. Keep in mind that when structure are drawn, hydrogen atoms are not always shown; you will sometimes need to infer them. See if you can find all the chiral centers in the atoms below:

Assigning Priority to Substituents

Now that we have found our chiral center(s), we need to assign the configuration to each. First, we need to assign priority to the four different substituents at each of the chiral centers. This is achievable using the Cahn-Ingold-Prelog system. The steps are:

  • Starting at the chiral center, the atom with the highest atomic mass bonded to the chiral center has the highest priority.
  • If two atoms are tied for priority, look at the next atom away from the chiral center. Determine priority of the substituent based on this atom. Always move towards the heaviest atom if possible.
  • Repeat until a difference in atoms is observed.
  • A double bond or a triple bond can be treated as a bond to two or three separate carbon atoms for this purpose.

Lets try some examples:

numbering substituents

Determining the Configuration

Unfortunately, this can be the tricky step as it requires you to think in 3D. We need to rotate the molecule in our heads so that the lowest priority substituent is facing away from us (into the page) and so the first priority substituent is pointed up. However, its not that bad; let’s practice with a few examples:

orienting molecules so first priority substituent is on top

Now, observe the position of the 1st, 2nd, and 3rd priority substituents. If they appear in order clockwise, you have the R isomer of the molecule. If they appear in order counter clockwise, you have the S isomer of the molecule.

Determining the configuration

Using Dashed and Wedged Bonds in Determining R/S Configuration

Wedged and Dashed bonds give information about the 3D shape of a molecule, but it is important to understand how these wedged and hashed bonds translate into a R or S configuration. Wedged (solid) bonds indicated that a bond is pointing towards you (out of the page), while a dashed bond indicates a bond that is pointing away from you (into the page).

If the lowest priority substituent has a dashed bond, you are in luck, it is already pointing to the back. All you need to do is rotate the molecule so that the first priority substituent is pointing up, and then determine whether the substituents count up clockwise or counterclockwise to get the stereochemistry.

Determining the configuration if lowest priority substituent is dashed

If the lowest priority substituent has a wedged bond, you will need to rotate the molecule so that the wedged bond is now pointing into the page (like a dashed bond). Then assess the stereochemistry like normal.

Determining the configuration if the lowest priority substituent is wedged

If any other bond is a dashed or wedged bonds, be cognizant of which way the bond will point when you rotate the molecule so that the lowest priority substituent points back. Then, number the substituents like normal and assign the R/S configuration.

For More Help, Watch Our Interactive Video Explaining R and S Configurations!

R/S Configuration Practice Problems

Try the following Examples:

Practice Configuration Problems

Answers: R,R,S