Finding R and S for Chiral Centers

Key Questions

  • What does R configuration mean?

    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:

    ![http://www.nobelprize.org/](useruploads.socratic.org)

    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:

    • NH2 has priority 1 due to highest atomic number for N.
    • COOH has priority 2 due to the higher atomic number of O vs. H in CH3
    • CH3 has priority 3 as a result.
    • H has priority 4.

    Now, if you draw a circular arrow starting at NH2, going to COOH, crossing over H since it is in the back, and to CH3, then you would have gone clockwise.

    ![https://upload.wikimedia.org/](useruploads.socratic.org)

    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 H in the front and CH3 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 H in the back and CH3 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 180o in space), you would get S configuration A.

    Truong-Son N. · 1 · Sep 7 2015
  • What does S configuration mean?

    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.

    Mika · 1 · Dec 16 2014

Questions

Stereochemistry (R and S), Isomers, and Optical Activity