Single correct option
Three reactions are carried out on the butenes: (X) cis-but-2-ene is treated with and then with aqueous ; (Y) trans-but-2-ene is treated with cold, dilute, alkaline ; (Z) cis-but-2-ene is treated with followed by . The product in each case is butane-2,3-diol. Which statement is correct?
- A. X and Y are the same racemic mixture, whereas Z is the optically inactive meso isomer
- B. X is the meso isomer, whereas Y and Z are racemic mixtures
- C. X, Y and Z are all the meso isomer
- D. X and Z are racemic mixtures, whereas Y is the meso isomer
Show answer and solution
Answer: (A) X and Y are the same racemic mixture, whereas Z is the optically inactive meso isomer
Peracid epoxidation followed by acid hydrolysis is an overall anti-dihydroxylation (epoxide formation is stereospecific syn, ring opening by water is backside). Anti addition of two OH groups to a cis alkene gives the pair, a racemic mixture, so X is racemic. Cold alkaline is a syn-dihydroxylation via a cyclic manganate ester; syn addition to a trans alkene also gives the pair, so Y is the same racemate as X. is syn addition; syn addition to the cis alkene gives the isomer, which has an internal mirror plane (meso, inactive by internal compensation). So only option A is consistent. (Anti addition to the trans alkene or syn to the cis alkene would give meso.)
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