Chemistry - STEREO ISOMERISM Question with Solution | TestHub
If the compound Cabcd is assumed to be square-planar, then how many stereoisomers are possible ? What are the stereochemical relationships among them ? If each of the square planar structures assumes a pyramidal structure, then how many stereoisomers are possible and what is their stereochemical relationship ?
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Answer:
Solution:
Ans. Square-planar : 3 stereoisomers, all achiral, related as diastereoisomers (geometrical isomers). Pyramidal : 6 stereoisomers = 3 pairs of enantiomers.
Sol. Square-planar Cabcd : fix a at the top. The remaining three groups can be arranged around the other three corners in 3 distinct ways (b, c, d clockwise; b, d, c clockwise; c, b, d clockwise) :
(I) 
(II) 
(III) 
Since all three are planar, none of them is chiral; they differ in which groups are adjacent (cis) or opposite (trans), so they are diastereoisomers (geometrical isomers) of one another.
When each square-planar structure is converted into a pyramidal structure with C at the apex, the central atom becomes a stereogenic centre with four different groups, and each arrangement gives a pair of enantiomers. Three pairs of enantiomers are formed, that is 6 stereoisomers; members of different pairs are diastereomeric.
