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In some electrophilic addition reactions, such as those with HBr or H2SO4 and an alkene, there is a choice as to which carbon ends up bonded to which atom. With Br2 there is no choice as both atoms are equivalent.
When HBr reacts with a nucleophile, it is attacked at the hydrogen and loses a bromide ion. The reaction produces a carbocation and this carbocation then rapidly reacts with the bromide ion, so that overall, the HBr has been added across the double bond.
For unsymmetrical alkenes remember to look for the most stable carbocation as this will predict the correct product. Tertiary carbocations are the most stable and primary carbocations are the least stable.