The emboldened hydrogen atom (shown in red below) is first replaced
by a different group with a higher atomic number, but with a lower atomic
number than the
substituent of next highest priority attached to the prochiral centre.
It is convenient to use an isotope (deuterium) for this as shown below.
Next, the four substituents
attached to what is now a stereocentre are assigned an order of priority
using the CIP rules. Rules 1 and 5 allow H to be assigned priority 4 and
D priority 3. Rule 2 then allows the methyl group to be given priority
2 and the ethyl group priority 1. The group of lowest priority is at the
back of the molecule as drawn, and the other three substituents decrease
in priority in an anti-clockwise direction, so the stereocentre has the
(S)-configuration. Thus, the topism of the hydrogen atom shown in
red is pro-S.
The emboldened hydrogen atom (shown in red below) is first replaced
by a different group with a higher atomic number, but with a lower atomic
number than the
substituent of next highest priority attached to the prochiral centre.
It is convenient to use an isotope (deuterium) for this as shown below.
Next, the four substituents
attached to what is now a stereocentre are assigned an order of priority
using the CIP rules. Rules 1 and 5 allow H to be assigned priority 4 and
D priority 3. Rule 2 then allows the methyl group to be given priority
2 and the ethyl group priority 1. The group of lowest priority is at the
back of the molecule as drawn, and the other three substituents decrease
in priority in a clockwise direction, so the stereocentre has the (R)-configuration.
Thus, the topism of the hydrogen atom shown in red is pro-R.
The emboldened methyl group (shown in red below) is first replaced by
13CH3. Next, the four substituents attached to what
is now a stereocentre are assigned an order of priority using the CIP rules
(see chapter 3 for details). The molecule is redrawn with the group of
lowest priority at the rear, and the other three groups decrease in priority
in a clockwise direction. Thus, the topism of the methyl group shown in
red is pro-R.
In this case, the molecule will be achiral even if the highlighted phenyl
groups is changed to a different group. Thus, the descriptors pro-R
and pro-S cannot be used.
The best way to describe the phenyl group shown in red below is as
pro-trans since if it replaced by a different group of higher priority,
then the two substituents of highest priority (Br and the modified phenyl
group) will be trans to one another on the six membered ring.
There are two ways of assigning the topism of the methyl group shown in red below. The simplest is to note that it is trans to both of the phenyl substituents on the five membered ring. Thus, its topism can be described as pro-trans.
Alternatively, replacement of the red methyl group by 13CH3
creates a pseudoasymmetric centre (cf. Chapter 4), and the absolute configuration
of this
pseudoasymmetric centre can be defined using the CIP rules as discussed
in Chapter 3. Rule 5 is used to distinguish between CH3 and
13CH3, and rule 6 is needed to distinguish between
the two halves of the five membered ring. The absolute configuration of
the pseudoasymmetric centre is r, so the topism of the red methyl
group is pro-r.
Replacement of the red hydrogen atom shown below by a deuterium atom
generates a trisubstituted alkene, the stereochemistry of which is best
described using the E / Z nomenclature. The two groups of highest priority
(D and Br) are on the same side of the alkene, so the alkene has the Z-geometry,
and the topism of the hydrogen shown in red is pro-Z.
Replacement of the red acid group shown below by 13COOH generates
a tetrasubstituted alkene, the stereochemistry of which is best described
using the E / Z
nomenclature. The two groups of highest priority (S and 13C)
are on opposite sides of the alkene, so the alkene has the E-geometry,
and the topism of the acid group shown in red is pro-E.
In this case, replacement of the red COOH group by 13COOH
generates a molecule which possesses a stereogenic axis. The absolute configuration
of the stereogenic axis can be defined by the CIP priority rules (cf. Chapter
3), and in this case is S. Therefore, the topism of the red acid group
is pro-S.
Replacement of the red coloured sulphur atom by 33S generates
an achiral molecule which possesses a pseudoasymmetric centre, the absolute
configuration of which is s. Therefore, the topism of the red sulphur
atom is pro-s.
Replacement of the red coloured hydrogen atom by deuterium generates
an achiral molecule which possesses a pseudoasymmetric centre, the absolute
configuration of which is s. Therefore, the topism of the red hydrogen
atom is pro-s.
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