First of all, nothing "forms" at the benzylic C atom, wrong choice
of words.
Secondly, your reasoning is totally wrong (ie. it's not about
electronegativity of O vs Br, because even if it was fluorine used
instead of bromine, and F is of course more electronegative than O,
the major and minor products discussed above will still be similar,
albeit with *additional* possible beyond-H2-syllabus products due
to the strongly oxidizing and highly reactive nature of
fluorine).
Draw the full mechanisms to get both major & minor products
(actually, there'll be a total of 3 possible products excluding
stereochemistry when using aqueous halogen, can you figure out
why?), and upload the image.
Note : many Singapore JCs teach wrongly that OH- attacks the
carbocation, but it is actually H2O that is the competing
nucleophile. Don't make this error in your mechanism.
Is it br and oh,
Oh and br,
And oh , oh?
