Anti addition

As a bromine molecule approaches perpendicular to the negatively charged pi cloud of the carbon-carbon double bond, its bonding electrons are repelled away from the bromine atom nearer the double bond, leaving it with a partial positive charge. As the positively charged bromine penetrates the pi cloud, a negative bromide ion breaks away from it, leaving a cyclic bromonium ion, in which the positive bromine is boned to two carbon atoms. Backside attack on the bromonium ion by a bromide ion results in a trans products. The bromine is added to opposite sides of the carbon-carbon double bond.