Finding R and S for Chiral Centers

Key Questions

• It is a stereochemical label to indicate the relative spatial orientation of each atom in a molecule with a non-superimposable mirror image.

R indicates that a clockwise circular arrow that goes from higher priority to lower priority crosses over the lowest priority substituent and that lowest-priority substituent is in the back.

The R and S stereoisomers are non-superimposable mirror images, which means if you reflect them on a mirror plane, they do not become the exact same molecule when you overlay them.

When you label a molecule as R or S, you consider the priorities of each substituent on the chiral carbon (connected to four different functional groups).

Let's take this chiral amino acid for example:

Some general ways you could determine the priorities are:

1. HIgher atomic number of the directly-attached atom gives higher priority
2. Atomic number of the atom attached to the one is considered in step 1 if two substituents have the same first atom
3. Higher number of same-atom branches determines greater priority if the overall substituents are too similar (e.g. isopropyl has higher priority than ethyl)

With (R)-alanine:

• ${\text{NH}}_{2}$ has priority 1 due to highest atomic number for $\text{N}$.
• $\text{COOH}$ has priority 2 due to the higher atomic number of $\text{O}$ vs. $\text{H}$ in ${\text{CH}}_{3}$
• ${\text{CH}}_{3}$ has priority 3 as a result.
• $\text{H}$ has priority 4.

Now, if you draw a circular arrow starting at ${\text{NH}}_{2}$, going to $\text{COOH}$, crossing over $\text{H}$ since it is in the back, and to ${\text{CH}}_{3}$, then you would have gone clockwise.

Since the lowest priority atom is in the back, the clockwise arrow corresponds to the R configuration.

If you had started from the same R configuration but oriented $\text{H}$ in the front and ${\text{CH}}_{3}$ in the back, it would have been S configuration. Let's call this S configuration A, where you just nudge two substituents to flip them from front/back to back/front.

If you reflect the same R configuration over a mirror plane, keeping the orientations of $\text{H}$ in the back and ${\text{CH}}_{3}$ in the front after the flip, the configuration is also S. Let's call this S configuration B, where you've actually done a reflection.

If you start from S configuration B, and flipped it over a vertical axis (literally rotating ${180}^{o}$ in space), you would get S configuration A.

• R and S are used to describe the configuration of a chirality center. Chirality center meaning that there are 4 different groups attached to one carbon. To determine whether the chirality center is R or S you have to first prioritize all four groups connected to the chirality center. Then, rotate the molecule so that the fourth priority group is on a dash (pointing away from you). Finally, determine whether the sequence 1-2-3 is (R) clockwise or (S) counterclockwise. Hope this helps.