Question

45. What is (are) the major product(s) obtained from the following reaction? L Br2 - L (2R, 3R)-Dibromobutane (2S, 3S)-Dibromobutane (2S, 3R)-Dibromobutane Ι A) Only I B) Only II C) Only III D) Only I and II

Answer 1

45. OPTION D. When a cis-alkene undergoes bromination, the reaction primarily involves the attack of a bromine molecule to form a bromonium ion (three membered ring with two carbons and one bromine bearing a positive charge). The stereochemistry of the product is determined solely by the face of attack of the bromide ion, in the face "anti" (opposite) to the bromonium ion. With two similar carbons, there exists equal probability of both carbons being attacked by the incoming bromide nucleophile. This attack then cleaves the bromonium ion to give the final vicinal dibromide.

Owing to the equal probability of attack on either of the carbons, the products formed will have equal quantities of both the enantiomers of the dibromide which will render the solution, on the whole, racemic. In terms of absolute configurations, both (2R, 3R) and (2S, 3S) molecules will occur in equal quantities.

Similarly, when a trans-alkene is brominated, the resulting product will be a meso-compound due to the same mechanism.