Question

Give the structure of the alkene ($C_{4}H_{8}$) which adds on HBr in the presence and in the absence of peroxide to give the same product, $C_{4}H_{9}Br$.

Answer 1

The structure of alkene is isobutylene is in the figure. It has three carbon atoms and methyl group have double bonded to the center carbon atom.  

The addition of HBr to isobutylene in the presence and absence of peroxide is given below:

In the absence of peroxide: the bromine atom bonds with the center atom by breaking the double bond.

In the presence of peroxide: the bromine atom bonds with the extreme atom and also breaking the double bond.