[iScience]

i don't understand what drives the initiation process.

initially, when the bromine comes close to the double bond can someone please tell me step by step what happens and why?? i'm very confused

[zsinger]

Electron rich double bond attacks Bromine, which, when it gives up its electrons to the other bromine, is Br+.  Plus attracts Minus………Hope that helped .
        Z

[iScience]

every time i've had an example with markovnikov addition reactions, i've always seen an H⁺ getting reduced. this makes sense because there is a high difference in electronegativity, ie there is a high differnece in potential, so i understand why the electrons went where they went. but with this, why are the electrons reducing a more electronegative species in the first place?

[zsinger]

I believe it is a "temporary" dipole moment which initiates the rxn.  I am not 100% sure though.

[orgopete]

I believe one should seek an atomic consistency to explain reactivity. Therefore, one can look at the atomic properties in other reactions as well. We know bromine is one of the most electron withdrawing atoms from the acidity of HBr or the SN1 reactivity of tertiary bromides. If we now combine to have two bromine atoms to share a pair of electrons, we may anticipate Br2 to be very electrophilic.

An example of Br2 reacting as an electrophile is to react with bromide to produce Br3(-). I would argue the reaction of an alkene should be more like the reaction of bromide than the six electron mechanism found in many books. You may find this intermediate written in some places, but I cannot remember where right now. (One might also reason the 2+2 addition of a bromine atom to be thermally forbidden.) Furthermore, consistent with an electrophilic reaction is the more electron rich an alkene, the faster the reaction. (If someone could post relative rates and a source, I'd appreciate it.)

If one writes a direct addition of a pair of electrons to Br2, then it becomes more difficult to convince everyone how or that a bromonium ion intermediate should form. I am not going to argue what intermediates might form and the electron movements to accomplish them. I think six electron mechanism posted is an expedient method to give a bromonium intermediate than a mechanism that explains why the reaction should occur. I simply think we should anticipate the alkene donates electrons to an electrophilic Br2.