Stereochemistry: Stereocenter Nomenclature: The R/S System
Exercise Solutions


Priority of groups on stereocenter: HO > CH3CH2 > CH3 > H.  The stereocenter has the S configuration.


Priority of groups on stereocenter: HS > CH(CH3)2 > CH2CH3 > H.  The stereocenter has the S configuration.

Priority of groups on stereocenters: HO > CHOH > CH3 > H.  Both stereocenters have the R configuration.

Priority of groups at amine stereocenter: NH > HC=C > CH(CH2)2 > H.  This stereocenter has the S configuration.
Priority of groups at alcohol stereocenter: HO > CH2C=O > CH3 > H.  This stereocenter has the R configuration.

Recall that an alkene carbon counts as being attached to two carbons, and an alkyne carbon counts as being attached to three carbons.  In this case the sp2 and sp carbons directly attached to the stereocenter both count as having three carbons attached, so we must move out one atom further.  The far carbon of the alkene has three four carbons attached (one for the methyl and three for the alkyne).  The far carbon of the alkene has two hydrogens and two carbons attached (two for the alkene).

Thus the alkyne has higher priority than the alkene.  Priority of groups at stereocenter: ester > alkyne > alkene > CH3.  This stereocenter has the S configuration.

f.  Labeling the stereocenters in a molecule of this complexity is best achieved by use of molecular models.  Remember to use the lowest priority group as a "handle" in the back to have the correct view of the molecule to determine the direction in which priorities decrease.

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