By the end of this guide,
You’ll become a pro at assigning R and S configurations—even when the fourth priority is on a tricky wedge! We’ll walk through each step with helpful tips and tricks to make the process easier, building skills that will save you time and boost your accuracy on exams. Let’s dive in and master this essential organic chemistry skill together!
Before we begin,
Let’s review what a chiral center is. A chiral center (also called a stereocenter, stereogenic center, or asymmetric center) is an sp3 hybridized carbon atom with four different atoms or groups bonded to it.
Fun Fact! Chiral centers are super important in organic chemistry because the way atoms are arranged around them can dramatically change a molecule’s physical, chemical, and biological properties.
For instance, (R)-limonene smells like oranges, while (S)-limonene has a lemon scent. In a more serious example, (R)-thalidomide was once used to treat morning sickness in pregnant women, but (S)-thalidomide tragically caused severe birth defects in the 1950s. This case highlights the critical need for enantiomer-specific testing in drug development.
So, how do scientists distinguish these stereoisomers and assign them as R or S? That’s exactly what we’ll uncover in this guide!
Challenge: The fourth priority is on a wedge!
Question:
Assign the configuration (R or S) of the chiral center in this compound?
Let’s walk through the steps together!
Step 1: Identify the four atoms attached to the chiral center
Look at the chiral center.
Can you identify the four groups attached? Take a moment to jot them down.
Step 2: Assign priorities based on atomic number
The atom with the highest atomic number gets the highest priority (1), and the atom with the lowest atomic number gets the lowest priority (4).
Question: Which atom/group has the highest atomic number? Which has the lowest?
Step 3: Resolve ties, if any
If any two atoms have the same atomic number, compare the next atoms along each chain until you find a difference.
Heads up! For this challenge, each atom attached to the chiral center is different, so we can skip this step. But we’ll tackle this rule in another example soon!
Step 4: Is the fourth priority on a dash?
In this challenge, the fourth priority is not on a dash. It’s on a wedge? which means it’s pointing out of the page, or “toward us.” This changes how we assign the configuration!
You have two options here:
The hard way: Rotate the molecule in your head so that the fourth priority is pointing away (on a dash). If you don’t have a model kit, this can be tricky!
The easy way (using a trick): Determine the configuration (R or S) based on the 1-2-3 sequence as you normally would, then invert it to get the answer. This saves you time, especially in an exam setting!
Step 5: Determine the direction of the 1-2-3 sequence
