Question

bromination mechanism

Answer 1

My question is, why doesn't Br-(black) attack? Why is it only Br-(green) that attacks? Both carbons are equally substituted.

Answer 2

At first, it is Br2 that attacks.
Such molecule is non-polar, made by Br-Br bond, with equal number of electrons on both sides.
As this molecules approaches the organic compound at the double bond side, it repells electrons, given that the Br2 also has electrons itself. After that, one of the ends of the double bond will have concentration of electrons and the other will not.
Similarly, during repelling, the same happens with bromine molecule.
THe repelling makes electrons go to the downmost Br and the upmost Br has lack of electrons.

Automatically, Br+ attacks the electron-concentrated area of the double bond.
Secondly, all you have left is the Br- that will attach the electron-deficient side of the double bond.