One or more options correct
Consider the addition reactions of the two isomeric but-2-enes. Select the correct statement(s).
- A. trans-But-2-ene reacts with in to give meso-2,3-dibromobutane.
- B. cis-But-2-ene reacts with cold, dilute, alkaline to give a racemic mixture of butane-2,3-diol.
- C. cis-But-2-ene, on treatment with a peracid followed by aqueous acid, gives meso-butane-2,3-diol.
- D. trans-But-2-ene reacts with followed by to give a racemic mixture of butane-2,3-diol.
Show answer and solution
Answer: A, D
Bromination is anti addition. For the trans alkene, anti addition of Br and Br across the planar double bond gives the (2R,3S) meso product, so A is correct. and hydroxylate by syn addition. Syn addition to the cis alkene gives the meso diol, so B is wrong; syn addition to the trans alkene gives the (R,R) and (S,S) pair in equal amounts, a racemic mixture, so D is correct. Peracid epoxidation retains the cis geometry, and acid hydrolysis opens the epoxide by anti attack of water, so the cis epoxide gives the racemic diol and not the meso diol; C is wrong.
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