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Consider the compound X: CH3-CH(OH)-CH=CH-CH(OH)-CH3. Which of the following statements about the stereoisomers of X are correct?

1. The total number of stereoisomers possible for X is 6
2. The total number of diastereomers possible for X is 3
3. If the double bond in X has trans (E) configuration, the number of enantiomers possible for X is 4
4. If the double bond in X has cis (Z) configuration, the number of enantiomers possible for X is 2

Correct answer: If the double bond in X has cis (Z) configuration, the number of enantiomers possible for X is 2

Solution

X has two chiral centres and a C=C double bond (E/Z). Total stereoisomers = 2 (E/Z) x 2² (chiral centres) = 8, but meso forms reduce this. The cis isomer has one meso form, giving fewer stereoisomers for that geometry.

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