Chemistry - STEREO ISOMERISM Question with Solution | TestHub
Discuss the stereochemistry of these compounds. (Hint. This means saying how many diastereoisomers there are, drawing clear diagrams of each, and saying whether they are chiral or not.)
(1) 
(2) 
(3) 
(4) 
Options:
Answer:
Solution:
Ans. (1) and (2): a single achiral compound each, no diastereoisomers. (3): two diastereoisomers - the cis ring junction is a meso compound (achiral), the trans is chiral (a pair of enantiomers). (4): two diastereoisomers (cis and trans ring junctions), both chiral.
Sol. Motive of the problem : making sure that you can handle this important approach to the stereochemistry of molecules. Just follow the hint! Diastereoisomers are different compounds, so they must be distinguished first; then it is easy to say whether each is chiral.
Compounds (1) and (2) are simple. The C=C double bond lies at the ring junction, so both junction carbons are sp2 and there are no stereocentres: one compound, no diastereoisomers, a plane of symmetry (the plane bisecting both rings through the oxygen), not chiral.
Compound (3) has the double bond on the far side of the six-membered ring, so the two ring-junction carbons are equivalent and each is a stereocentre. It may exist as two diastereoisomers: the cis ring junction has a plane of symmetry (a meso compound - not chiral), while the trans ring junction has no plane of symmetry but a C2 axis - it is chiral and exists as a pair of enantiomers.
Compound (4) has no symmetry at all (the double bond is unsymmetrically placed). Again there is a cis or trans ring junction, but this time both diastereoisomers are chiral - each exists as a pair of enantiomers (4 stereoisomers in all).
