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The Orientation of Electrophilic Substituents on Benzene Derivatives

The atom bound to the benzene ring carries a negative charge or (and) an electronic doublet

In this case, the doublet participates in the mesomerism of the cycle and generates negative charges in the ortho and para in the in the contributive mesomeric forms: Example: Phenol

Phenol thus carries partial negative charges in ortho and para positions. An electrophilic reagent X+ is therefore preferably attacking phenol in ortho and para positions.

The atom bound to the benzene nucleus carries a positive charge or (and) is the source of a readily displaceable double bond:

In this case, a doublet of benzene tends to prolong the mesomerism towards this atom and shows positive charges in ortho and para in the contributive mesomeric forms: Example: Benzoic acid

The benzoic acid thus carries partial positive charges in ortho and para positions. An electrophilic reagent X+ will therefore not attack at these positions, but preferably in meta . →    Exercises