Chemistry - STEREO ISOMERISM Question with Solution | TestHub
How many stereoisomers are possible for a molecule having the formula C(A*)4 where A* represents an asymmetric centre ? Are all of them optically active ?
Answer:
Solution:
Ans. 6 stereoisomers; no, not all are optically active.
Sol. With four asymmetric centres of the same kind attached to the central carbon, the configurations of the four arms can be enumerated as sequences of R and S, remembering that rotating the molecule permutes the arms :
(R,R,R,R) and (S,S,S,S) : a pair of enantiomers, both optically active.
(R,R,R,S) and (S,S,S,R) : a second pair of enantiomers, both optically active.
(R,R,S,S) with like configurations adjacent : has a plane of symmetry - meso, optically inactive.
(R,S,R,S) with like configurations alternating : also has a plane of symmetry - a second meso form, optically inactive.
Total : 6 stereoisomers = 4 optically active (two enantiomeric pairs) + 2 meso (optically inactive).
