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Question: When cis but-2-ene is treated with \(B{{r}_{2}}\) in \(CC{{l}_{4}}\) medium, the product formed will...

When cis but-2-ene is treated with Br2B{{r}_{2}} in CCl4CC{{l}_{4}} medium, the product formed will be
a) (2R, 3S) dibromobutane
b) (2R, 3R) ​dibromobutane
c) (2S, 3S) dibromobutane
d) mixture of (2R, 3R) and (2S, 3S) ​dibromobutane

Explanation

Solution

Reaction of alkene with halogens in CCl4CC{{l}_{4}} medium is an anti-addition reaction in which the halide ions will add opposite to the geometry of the reactant. In case of cis but-2-ene, halide ions will add in opposite faces as reactant is cis which means on same side.

Complete answer:
In this reaction, cis but-2-ene will act as nucleophile and attack on Br2B{{r}_{2}}which leads to the formation of non- classical carbocation. Since the medium is a non-polar solvent hence other parts of Br2B{{r}_{2}} will act as nucleophiles and attack from the opposite side of the non-classical carbocation and we will get our desired product.
-Attack by alkene on Br2B{{r}_{2}}:-

As we can see, alkene acts as a nucleophile and attacks the electrophile (Br2B{{r}_{2}}). This step is the rate determining step of the reaction.
-Formation of non-classical carbocation:-

As soon as carbocation forms on carbon, bromine gives its lone pair to it, hence forming the non-classical carbocation (which is not a complete carbocation).
-Attack of nucleophile to form our final product:-

Since no other nucleophile is present other than Br−B{{r}^{-}} ion, it can attack on both the carbon from the opposite side of the other bromine atom. This way the anti reaction takes place.
-The product formed is an enantiomeric pair as they are non-superimposable mirror images of each other. The pair is (2R, 3R) and (2S, 3S) dibromobutane.

Hence the correct answer is: d) mixture of (2R, 3R) and (2S, 3S) dibromobutane.

Note:
In the final solution both enantiomers will be present in equal amounts hence we will obtain a racemic mixture.
-If we are given polar solvent instead of non-polar solvent, than the ions of polar solvent will act as nucleophile and will attack the carbocation before Br−B{{r}^{-}}ion could, as in actual reaction it is weakly attached to the non-classical carbocation for a while due to which other ions get to attack easily.