CHAPTER 7: Question 2

3D 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 3D structure shown below illustrates the equivalence of the two OMe and two hydrogen atoms attached to the central carbon atom.

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.

3D representations of the three compounds shown above are given below.

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.

3D representations of the four compounds shown above are given 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.

The 3D structure shown below may aid in the visualization of this compound.

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 3D structures shown below may be helpful in visualizing the symmetry in the two species.

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. 3D Diagrams of each of the stuctures are also given 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. 3D Diagrams of each of the stuctures are given below.

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. The 3D structure shown below illustrates this.

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. 3D structures of both the original and modified structure are shown below to illustrate the prostereogenic axis and stereoaxis.

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. 3D structures of both the original and modified structure are shown below to illustrate the prostereogenic axis and stereoaxis.

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. The 3D structure shown below illustrates this.

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. 3D representations of these four structures are shown below to emphasize the various prostereo- and stereo-features.

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. The 3D structures may emphasize the stereo and pro-stereo features in these structures.

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. The 3D structures may emphasize the stereo and pro-stereo features in these structures.

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