The diastereomer of A can be obtained by exchanging the positions of the hydrogen and deuterium atoms as shown below. The most stable conformation of this compound will again have the two phenyl groups antiperiplanar to one another, but in this case the deuterium atom rather than the hydrogen atom is in the correct location for a pyrolytic elimination. Thus, this compound would give the (E)-isomer of 1,2-diphenylethene which contains no deuterium as shown below.
The enantiomer of A would give exactly the same product (B) as obtained from A. This can be seen in the diagram below, but should also be obvious since enantiomers have identical chemical properties. Thus, if one enantiomer of a starting material gives a particular achiral product then the other enantiomer of the starting material must give the same achiral product.
The reactions are stereoselective since a single stereoisomer (cis-trans
isomer) of the product is obtained from each stereoisomer (enantiomer or
diastereomer) of the starting material. However, the reactions are not
stereospecific since the diastereomer of A gives a product which is not
a stereoisomer of the product obtained from A. In particular, the product
from A contains deuterium whilst that from the diastereomer of A does not.
Similarly, A and its enantiomer give identical rather than stereoisomeric
products.
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