CHAPTER 4: Question 3

3D explanations

The first compound contains two stereogenic axes as shown by the red and blue bonds below. Thus, there are a maximum of 4 stereoisomers of the compound. However, the substituents attached to the two stereogenic axes are identical, so there are only three stereoisomers represented by structures A-C (and the three 3D structures) below. Note that in the case of the terminal aryl groups, it is the bromine atom in the meta position which provides the difference between the two sides of the aromatic rings, rather than the methyl groups in the ortho positions. Structures A and B are a pair of enantiomers (and so will both be optically active), and structure C is an achiral meso compound which is a diastereomer of both A and B, and which is optically inactive. The analysis of this compound is identical to that of a compound which contains two stereocentres, each with identical substituents.

 

To address the second compound, it is first necessary to recognise that this is a spiro-bicyclic compound, and hence as discussed in Chapter 3 it contains a stereogenic axis. The location of the stereogenic axis is indicated by the red line in the diagram below. Thus, the compound contains one stereogenic axis and two stereocentres (at the carbon atoms shown in blue below), giving a maximum of eight stereoisomers. These eight structures are shown below, however for a stereogenic axis is it irrelevant which end is considered to be the front and which the rear, so the boxed structures are identical (they can be interconverted by a 90o rotation about an axis perpendicular to the screen followed by a 180o rotation about a vertical axis). Thus, there are only six stereoisomers of this compound. Isomers B and D are a pair of enantiomers, as are isomers A / E and C / F. The relationship between any other pair of isomers is that they are diastereomers, and all of the compounds are optically active since none of them are meso compounds. 3D structures corresponding to each isomer are also given below, to allow the relationship between them to be visualized more readily.


 
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