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 alkyne has 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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