Question

Cis-2-butene on reaction with $B{r_2}$ in $CC{l_4}$ produces mainly:
A. 1-bromo-2-butene
B. 2,3-dibromobutane
C. Meso-2,3-dibromobutane
D. $( \pm )$2,3-dibromobutane

Answer 1

Addition reaction is a reaction in which a substrate is initially attacked by an electrophile (since double and triple bonds are considered as nucleophile), and the overall result is the addition of one or more relatively simple molecules across a multiple bond.

Complete answer:
Cis-2-butene is an alkene. Alkenes are considered as nucleophiles. All alkenes undergo an additional reaction to the double bond. Because alkenes are nucleophiles, electrophiles attack first and get attached to the molecule. Now the resulting molecule will have a carbon - carbon double bond and a positive charge on one of the carbon. Now the molecule becomes carbocation that means an electrophile. Now the attack of the nucleophile will occur at the carbocation. It produces a molecule with a single bond between the carbon and carbon and two new bonds formed. So in the addition reaction for an alkene, one bond breaks and two new bonds form.
Now treatment of alkenes with bromine gives vicinal dibromide. The bromine adds to opposite faces of the double bond. That clearly means bromine addition will be anti-addition.
The double bond at the alkene is planar. Attack of the bromine on alkene gives the Bromonium ion, and carbocation. Bromonium ion then attacks from the backside of the carbocation because of the steric hindrance. Bromines are delivered at the opposite face of the alkene (anti-addition) so the 1:1 of enantiomers will form in this reaction.

**Hence the correct option will be D. $( \pm )$2,3-dibromobutane.

Note:**
$CC{l_4}$ actually does not affect the addition reaction of bromine to the alkene. It only acts as a solvent but when water or alcohol acts as a solvent different products will be formed, not dibromobutane.