CHAPTER 7: Question 2
2D Explanations
The sequence of the answers is from left to right along each row in turn.
This compound contains no prostereogenic features. The carbon atom of
the CH2 group is not a prostereogenic centre since replacement
of one of the hydrogen atoms or one of the two OMe groups attached to this
carbon atom by a different group still leaves two identical substituents
attached to the carbon atom, and so does not create a stereocentre. Similarly,
the carbon atoms of the CH3 groups are not prostereogenic centres
since replacement of one of three hydrogen atoms attached to the carbon
by a different group still leaves two hydrogen atoms attached to the carbon
atom and so does not create a stereocentre.
The two carbon atoms shown in red in the diagram below are prostereogenic
centres and are prochiral centres since replacement of one of the hydrogen
atoms attached to these carbon atoms by a new group (D) converts the carbon
atom into a stereocentre and forms a chiral molecule as illustrated below.
Note that the central CH2 group is not a prostereogenic centre
since it is attached to two hydrogen atoms and two ethyl groups, so replacement
of any one group by a new substituent will still leave two identical substituents
and so will not create a stereocentre.
The two carbon atoms shown in red in the diagram below are prostereogenic
centres and are prochiral centres since replacement of one of the hydrogen
atoms attached to these carbon atoms by a new group (D) converts the carbon
atom into a stereocentre and forms a chiral molecule as illustrated below.
The central carbon atom (shown in blue in the diagram below) is also a
prostereogenic centre and a prochiral centre since replacement of either
of the ethyl groups attached to this carbon atom by a new substituent (X)
converts the carbon atom into a stereocentre and forms a chiral molecule
as illustrated below.
This compound contains no prostereogenic features. You should first
note that the compound contains two stereocentres, and these both have
the (S)-configuration. Therefore, the carbon atom of the CH2
group is not a prostereogenic centre since replacement of one of the hydrogen
atoms or one of the two CHBrMe groups attached to this carbon atom by a
different group still leaves two identical substituents attached to the
carbon atom, and so does not create a stereocentre. Another way to approach
this problem is to note that the molecule possesses a C2
axis as indicated below, and rotation around this C2
axis interconverts the locations of the two hydrogen atoms and the locations
of the CHBrMe groups. Thus, the hydrogen atoms are homotopic as are the
CHBrMe groups, and the central carbon atom cannot be a prostereogenic centre
since it is not attached to any heterotopic substituents.
The carbon atoms of the CH3 groups are not prostereogenic
centres since replacement of one of three hydrogen atoms attached to the
carbon by a different group still leaves two hydrogen atoms attached to
the carbon atom and so does not create a stereocentre. The carbon atoms
of the CH groups cannot be prostereogenic since they are already stereocentres.
This compound is the diastereomer of the previous case, but this changes
the analysis. The carbon atom of the CH2 group (shown in red
below) is now a prostereogenic centre since the two CHBrMe groups are no
longer identical as they have opposite absolute configurations. Hence,
replacement of one of the hydrogen atoms attached to the CH2
group by another substituent (X) results in four different substituents
being attached to the red carbon atom. In this case however, the red carbon
atom is converted into a pseudoasymmetric centre rather than a stereocentre,
so the carbon atom of the CH2 group is a prostereogenic centre
but is not a prochiral centre. Note that the product of this transformation
is achiral since it contains a plane of symmetry.
The carbon atoms of the CH3 groups are not prostereogenic
centres since replacement of one of three hydrogen atoms attached to the
carbon by a different group still leaves two hydrogen atoms attached to
the carbon atom and so does not create a stereocentre. The carbon atoms
of the CH groups cannot be prostereogenic since they are already stereocentres.
This compound is very similar to the preceding two examples. Note first,
that the two stereocentres have the same absolute configurations. Therefore,
the carbon atom of the central CH2 group is not a prostereogenic
centre for the same reasons as discussed in the fourth example. The carbon
atoms of the CH groups are also not prostereogenic centres since they are
already stereocentres. This leaves the carbon atoms of the remaining two
CH2 groups (shown in red below) to consider, and both are prostereogenic
centres and prochiral centres since replacement of one of the hydrogen
atoms attached to these carbon atoms by another substituent (D) converts
the carbon atom into a stereocentre and forms a chiral molecule as illustrated
below.
This compound is a combination of the previous two examples. Thus,
the carbon atom of the central CH2 group (shown in red below)
is a prostereogenic centre but not a prochiral centre since it is converted
into a pseudoasymmetric centre when one of the two hydrogen atoms is replaced
by a different substituent (D). This change does not however create a chiral
molecule, so the prostereogenic centre is not a prochiral centre. The carbon
atoms of the other two CH2 groups (shown in blue below) are
both prostereogenic centres and prochiral centres since replacement of
one of the hydrogen atoms attached to the carbon atoms by another substituent
(D) converts the carbon atom into a stereocentre and forms a chiral molecule.
None of the atoms in this molecule can be a prostereogenic centre since
only the carbon atoms of the CH3 groups are attached to four
other atoms, and since three of these are identical, replacement of one
of the hydrogen atoms by another substituent will not create a stereocentre.
However, the possibility of a prostereogenic axis must also be considered
since allenes have the potential to possess a stereogenic axis. The requirement
for a stereogenic axis is that the two substituents attached to each end
of the axis must be different. In this case, the two substituents at the
ends of the axis (two hydrogen atoms at one end and two COMe groups at
the other) are both identical. Hence, replacement of any one of these substituents
by a different group will not create a stereogenic axis, so the molecule
possesses no prostereogenic features.
As in the previous example, this compound contains no prostereogenic
centres. In this case however, the molecule does posses a prostereogenic
axis as shown below, since changing one of the COMe groups to another group
(X) creates a stereogenic axis. The prostereogenic axis is also a prochiral
axis since the product obtained when one of the COMe groups is converted
into an X group is a chiral molecule.
As in the previous example, this compound contains no prostereogenic
centres, but does posses a prostereogenic axis as shown below. Thus, changing
one of the
hydrogen atoms to another group (D) creates a stereogenic axis. The
prostereogenic axis is also a prochiral axis since the product obtained
when one of the hydrogen atoms is converted into a deuterium atom is a
chiral molecule.
As in the previous three examples, this molecule does not posses any
prostereogenic centres. In this case however, the C=C=C axis is already
a stereogenic axis and so cannot be a prostereogenic axis. Thus, this compound
does not contain any prostereogenic features.
In this case, the carbon atoms of the CH2 and C(Me)2
groups (shown in red) are both prostereogenic centres and prochiral centres
since changing one of the hydrogen or methyl groups to a new substituent
(D or X as shown below) converts the carbon atoms into stereocentres and
creates a chiral molecule. The C=C=C axis is also a prostereogenic and
prochiral axis since changing one of the hydrogens attached to the axis
to a deuterium atom forms a stereogenic axis and creates a chiral molecule.
This molecule does not posses any prostereogenic centres since none
of the atoms are attached to four other atoms. However, the lower aromatic
ring is a prostereogenic plane and a prochiral plane since replacement
of any of the hydrogen atoms (ortho- or meta- to the acid group) on this
ring by a deuterium atom creates a stereogenic plane and forms a chiral
molecule as illustrated below.
This molecule does not posses any prostereogenic centres since none
of the atoms are attached to four other atoms. However, both aromatic rings
are prostereogenic planes and prochiral planes since replacement of any
of the hydrogen atoms (ortho- or meta- to the substituents) on the rings
by a deuterium atom creates a stereogenic plane and forms a chiral molecule
as illustrated below.
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