Bromination of an alkene in presence of water results in its mechanism in a cyclic bromonium ion and "afterwards $\ce{H2O}$ binds to the carbon atom surrounded by more electron donating groups." This is what we learned in class and I couldn't understand why. Shouldn't an electron donating group decrease the "strength" of an electrophile?

Also to my surprise, in an exercise, $\ce{H2O}$ was added to the formerly $sp2$ carbon bonded to $\ce{-OCH3}$ which donates electrons by resonace in an alkene. But $\ce{H2O}$ was added to the cyclic bromonium and not to the alkene directly! So why to care about the resonance effect of $\ce{-OCH3}$?

  • $\begingroup$ Electron donation stabilises the carbocation so that it hangs around long enough to react with a nucleophile $\endgroup$ – Waylander Sep 20 '19 at 13:47
  • $\begingroup$ But carbocations are not formed in halogenation mechanism of alkenes $\endgroup$ – user208973 Sep 20 '19 at 13:54
  • $\begingroup$ they are formed as an equilibrium form of the cyclic bromonium ion, particularly if stabilised $\endgroup$ – Waylander Sep 20 '19 at 14:25

While there is no formal carbocation intermediate, the cyclic bromonium intermediate has a few different resonance forms (reference):

enter image description here

As you can see on the right, in the structure representing the resonance hybrid, partial positive charges build up on the electrophilic carbons connected to the bromine. The carbon that is more substituted with electron donating groups has a greater partial positive charge (and therefore more electrophilic character) because of the stabilizing effect of the substituents. That's why the nucleophile attacks at the more substituted carbon.


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