Discussion
Number of stereoisomers possible for the compound, CH3CH = CH-CH(OH)CH = CHCH3 is :
Answer: A
The compound has 4 stereoisomers.
CH3CH=CH-CH(OH)-CH=CHCH3 has two stereogenic double bonds (E or Z) and the carbinol carbon. Because the molecule is symmetric, the two halves are identical when both double bonds have the same geometry:
- (E,E): the carbinol carbon carries two identical E-propenyl groups, so it is not a stereocentre: 1 isomer.
- (Z,Z): likewise 1 isomer.
- (E,Z): the two propenyl groups now differ, so the carbinol carbon is a stereocentre: 2 enantiomers.
That gives 1 + 1 + 2 = 4. The formula 23 = 8 overcounts because it treats the molecule as unsymmetrical, and 6 or 7 come from partial corrections for the symmetry.
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