Explain why the addition of HI to 3,3-dimethylbut-1-ene gives 2-iodo-2,3-dimethylbutane as the major product and not the 2-iodo-3,3-dimethylbutane.

Explanation

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)`