• Also note that ethene is not usually reacted with $\ce{HBr(aq)}$. Ethene is usually reacted with $\ce{HBr(g)}$, in which case the acid is not ionized. In this case, the electron-rich (nucleophilic) part of ethene (the $\pi$ bond) reacts with the electron poor (electrophilic) part of $\ce{HBr}$ (the hydrogen atom). – Ben Norris Nov 5 '14 at 12:00