
When cis but-2-ene is treated with in 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
Answer
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Hint: Reaction of alkene with halogens in 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 which leads to the formation of non- classical carbocation. Since the medium is a non-polar solvent hence other parts of 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 :-
As we can see, alkene acts as a nucleophile and attacks the electrophile ( ). 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 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 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.
Complete answer:
In this reaction, cis but-2-ene will act as nucleophile and attack on
-Attack by alkene on

As we can see, alkene acts as a nucleophile and attacks the electrophile (
-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
-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
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