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Size:  128.5 KB For this question, I don't understand why Br+ is formed rather than Br- ? I get that this is electrophilic substitution reaction...

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    (Original post by coconut64)
    Name:  g.png
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Size:  128.5 KB For this question, I don't understand why Br+ is formed rather than Br- ? I get that this is electrophilic substitution reaction...

    Thanks
    Think about the C-Br bond that is breaking as the bromine leaves.

    The two electrons that were in this bond both move into the benzenoid ring so that it gets its aromatic 6electron count.

    That bond was covalent, so that means that the bromine atom 'contributed' one of those two electrons, but now the electron bromine contributed to the bond has been taken into the benzenoid ring!
    So Bromine has lost an electron (compared to being a bromine atom) so it must be Br+.

    Another way to thing of it, is that, in each step, the overall charge of all your reactants must be constant, to begin with you have overall +1 charge on the nitronium ion.

    When it adds it gains electron density from the ring, so the +1 charge is now in the ring (in the second structure the NO2 functionality shouldn't have a charge).

    Then when the Bromine leaves you get a neutral ring, so the bromine must have a +1 charge, if Br- appeared and the ring was neutral, that would mean 2 electrons had appeared from nowhere!
 
 
 
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