Question

Why only ortho and para isomers can be easily separated by electrophilic substitution and not the meta isomers ?
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Answer 1

It's due to the resonance of the isomers. If we look into the rosonance, the electron density will be more in the Ortho and para position. Thus the electrophilic (electron loving groups) substitution will be more in the Ortho and para positions. In the meta position electron density will be less . So the reaction of the electrophilic substances will be low here. So , since the electrophilic substitution is more in Ortho and para position, Ortho and para isomers can be easily separated.