Question

cis-But`-2-`en on reaction with `Br_(2)` in `C CI_(4)` produces mainly
A. `(+-)-2,3-`dibromobutane
B. meso`2,3-`dibromobutane
C. `(+)-2,3-`dibromobutane
D. `(-)-2,3-`dibromobutane

Answer 1

Correct Answer - A
Addition of halogens to alkenes is steroselective (trans or anti) as well as sterospecific. The addition is also said to the anterafacial (i.e. groups of the addened bond form opposite faces of the double bond). Cis-But`-2`-en yields race`-2,3-`dibromobutane while trans-but`-2`-en yields meso`-2,3-`dibromobutane. Therefore this type of reaction is diastereospecific.
The first step is the formation of a bridged cation, called a bromonium ion. In the second step, the nucleophile `(Br^(-))` adds to the face away form teh bridging group to yield the anti-addition product.

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