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During second electrophilic substitution on benzene ring the position of entry of the incoming group is dictated by the electronic effects imparted upon by the group already sitting there. Methoxy (-OCH3) is an electron donating group (+I, as well as +M) and this additional electron density is delocalised through resonance toward the 2-, 4- and 6- positions with respect to it. Hence the incoming (second) group as E+ favours to enter into these positions i.e. o- and p- positions. The 3- 1nd 5- positions (m-) are not directly fed this extra el. density and as such reman a disfavoured destination for the incoming E+. Hence the result.
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