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How do you determine whether a molecule has R or S configuration? Watch

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    I'm a biochemistry student and my lecturer's really confusing. How do you tell whether a molecule has R or S configuration? Please use organic molecules (big ones like amino acids are appreciated) as examples. Thanks!
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    It's the Cahn-Ingold-Prelog notational system.

    1) Identify the four different atoms or groups attached to the stereogenic center.

    2) Rank the atoms or groups based on the priority rules:

    The priority rules rank atoms and groups based on atomic mass. The following list summarizes these rules.

    a) For the four atoms directly attached to the stereogenic center, the higher the atomic mass, the higher the rank.

    b) If two or more atoms directly attached to the stereogenic center have the same mass, work outward along the chains of the groups they are in, atom by atom, until a point of difference is reached. The rank is assigned at this point of difference, based on the difference in atomic mass.

    c) If a group contains multiple bonds, the doubly or triply bonded atoms are counted as two or three of those atoms, respectively. Thus the carbonyl group is considered to have two carbon-oxygen bonds, one actual and one theoretical.
    3) Orient a projection of the molecule in space so that the group or atom of lowest rank is eclipsed by the stereogenic center.

    4) If the ranking declines in a clockwise direction, the configuration is R; if the ranking declines in a counterclockwise direction, the configuration is S.

    Oh, and big up the 'ol IC!
 
 
 
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