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Chemistry LibreTexts

7.06: The Cahn-Ingold-Prelog R-S Notational System

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  • To indicate the absolute three-dimensional configuration about a chirality center, the letters R and S are used.


    Image: chirality


    Identify the chirality center

    rank the four atoms attached to the chirality center in order of their atomic mass

    largest atomic mass = priority 1

    second largest atomic mass = priority 2

    third largest atomic mass = priority 3

    smallest atomic mass = priority 4

    orient the lowest priority atom so that it points away from you


    1-2-3 clockwise: R

    1-2-3 counter-clockwise: S


    comparing groups of atoms – first point of difference

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