Reaction of 2,3 -dimethyl-1-butene with HBr leads to an alkyl bromide, $\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Br} .$ On treatment of this alkyl bromide with $\mathrm{KOH}$ in methanol, elimination of HBr occurs and a hydrocarbon that is isomeric with the starting alkene is formed. What is the structure of this hydrocarbon, and how do you think it is formed from the alkyl bromide?