Explain why the addition of Br_2 to but 3,3-dimethylbut-1-ene occurs across the double bond without rearrangement of the carbon skeleton while that of HBr gives a major product with rearranged carbon skeleton. Justify your answer.

Explain why the addition of Br_2 to but 3,3-dimethylbut-1-ene occurs a

1.	Explain why the addition of Br_2 to but 3,3-dimethylbut-1-ene occurs across the double bond without rearrangement of the carbon skeleton while that of HBr gives a major product with rearranged carbon skeleton. Justify your answer.
Answer» Solution :The addition of `Br_2` across the DOUBLE bond occurs through the CYCLIC bromonium ion intermediate (I) and hence rearrangement of the carbon skeleton does not occurs to GIVE addition product (II). In contrast, addition of HBR occurs through a carbocation intermediate. Since the initially formed `2^@` carbocation (III) being less stable rearranges by 1,2-methyl shift to form the more stable carbocation (IV) which then reacts with `Br^-`to give the rearranged addition product (V). `underset"(more stable)"underset(3^@ "Carbocation (IV)")(CH_3-underset+oversetoverset(CH_3)\|C-undersetunderset(CH_3)\|CH-CH_3)overset(Br^-)to undersetunderset"(V)""2-Bromo-2,3-dibromobutane"(CH_3-undersetunderset(Br)\|oversetoverset(CH_3)\|C-undersetunderset(CH_3)\|CH-CH_3)`