When the Br 2 molecule approaches one face of the double bond, the π electron density 'pushes' electron density in the Br 2 molecule away from the closer Br atom, giving it a partial positive charge, ...When the Br 2 molecule approaches one face of the double bond, the π electron density 'pushes' electron density in the Br 2 molecule away from the closer Br atom, giving it a partial positive charge, and towards the more distant Br atom, giving it a partial negative charge. The bromine anion then attacks one of the carbons in the organobromoniom ion from the backside, opening the ring and completing the trans addition process.
